I once had a situation as you described, and it eventually turned out to be the power cable - there was a bad connection somewhere in/around the wall plug. It took months to track it down, but at least it was cheap to fix!

Guys, I did the memtest86+, and it didn't report errors. So now we are sure that the memory is not the problem. But We've checked the Motherboard, Memory and the temperatures, and now? What hardware may be causing those shutdowns?

Pastychomper, tanks, I'll replace it and will check if the problem will occur.

I think it is the PSU, how much power does it provide and is it enough:
http://www.extreme.outervision.com/p...ulatorlite.jsp

H_TeXMeX_H I don't think it is the PSU, when I bought this PC, 2 years ago, I wore the same hardware that I have now + nvidia geforce 9500GT, and my PSU was 500W, my video card broke, so I removed and it still working with that PSU until now, when it started to shut itself down I bought this new PSU "CORSAIR CX500" 500W as well.So, the system never shut itself down when I had a graphic card, why the PSU can't provide the enough energy anymore? I did the simulation in the site that you wrote, and I got this.

Minimum PSU Wattage: 274 W

Recommended
PSU Wattage: * 324W

My desktop configuration is: CPU: AMD PHENOM II X4 3.2GHz BLACK EDITION, MARK VISION 4GB DDR3, HD: 500GB 5400RPM, GPU: Nvidia 7025a (onboard), DVD drive, 1 80mm fan.

Try this CPU test in mode 1:
http://www.mersenne.org/freesoft/#source
If it passes for 13 rounds, then the only thing left is the motherboard if the PSU is new and known good. Do you have a spare mobo to test with ?

Unfortunately, I don't have another motherboard to test it. How do I start the program? I tried to execute "./prime.c" "./prime.h" but it didn't works. Tanks

someone?

You don't have to compile it, there are binaries as well:
ftp://mersenne.org/gimps/

Run it and then answer N=Just stress testing, press enter on the threads question, and then the torture test number is 1 for only CPU testing (plus a bit of RAM).

Have you ever tried disconnecting all cables from the motherboard and reconnecting them?
Recently I had a similar issue, so eventually I replaced the motherboard and everthing was ok. Out of curiosity I put the old "broken" motherboard back in and it worked as brilliant as it could be. Unfortunately I still don't know what it actually was but it still works...

That's actually good advice for a lot of things dt64: check the connections first. Fixed my car more than once by doing that. These new lead-free motherboards can actually grow "tin spurs" that'll bridge gaps between contacts. Unplugging them and plugging them back in just scrapes the connector clean but it works. CRC QD (quick dry) electronic cleaner might help too... just in case you got some pizza grease and coke in there. :o LOL

(After writing this, I see that some other posts already have covered some of this.)

Power supplies have automatic shutdown on several criteria. Which exact conditions will trigger a shutdown should be listed on the box and manuals.
Overvolt : one of the outputs has exceeded a valid voltage.
Overcurrent : one of the outputs is drawing an excessive amount of current.
Undervolt : insufficient mains voltage

In addition the motherboard can command the PSU to shutdown. This is a signal wire in one of the PSU motherboard power connectors. If those wires has a bad connection then any vibration can cause the PSU to shutdown.

1. Disconnect and remove drives, video card, PSI cards, and USB equipment from the motherboard.
2. Turn on PSU (with just motherboard connected), and note if it will run.
I assume the front panel power switch is still connected. (see post below for alternative PSU power on/off.)

If it does shutdown now, then
2a. check the power going into PSU as being valid voltage. Sometimes a transformer on the pole goes bad and you only get 90 Volts. If low then check against PSU requirements.
2b. Wiggle the cables between the motherboard and PSU. If it does shutdown when cables are wiggled, then either broken wire, bad solder joint in PSU, or bad connector
2c. Bad power switch to motherboard, or bad connector to faceplate switch. A short here could switch off power. Check that the power switch wires are actually on the correct pins.
Cross connecting the power switch wiring to something else could cause this kind of problem.
A broken faceplate power switch may create false button press.
Tap on (but not push) the power switch, front and backside, the switch terminals, and its wiring. If it does shutdown then replace that switch (or wiring). You can tap with something non-conductive. (see post below for alternative PSU power on/off.)
2c. Otherwise, suspect excessive current draw by motherboard. This can be due a voltage regulator, or CPU failure, or northbridge failing. Feel around for something excessively hotter than the normal hot. Dampen your fingers (not wet, just damp enough so you don't get burned) and quickly tap heatsinks and tops of chips. If your finger sizzles then that is it. Don't get water in connectors or on chip pins.
2d. Overclocking will make some chips run hotter than normal, and will increase current drawn from PSU.
2e. Note. some recent designs may normally run so hot that your finger sizzles. You must know if you have one of these kind of extreme boards. They have large cooling systems that MUST be working, and probably require the covers in place to cool things adequately. The PSU must be choosen appropriately for such a system.


3. PSU does not shutdown with only motherboard.
So add back components one at a time, and testing for PSU shutdown.
Video card.
Hard drives, one at a time.
Sound cards.
If shutdown with card in place, then
3a. Bad card. Check for excessively hot device.
3b. Mostly likely the PSU has insufficient current for the video card you have choosen. Many top end video cards have excessive current needs on particular cables. Many cheap PSU manufacturers will fudge their specifications. The wattage of the PSU is not the only important factor. Total up the current requirements needed for each voltage (motherboard, video card, drives, plus extra devices). An AMD system will have a different current specification than an Intel system, which can affect which PSU you choose. Your PSU must be able to supply that current at the specified voltages and more. Notice that there may be multiple independent voltage outputs on some PSU units (like two independent +12Volt outputs). Try to have 40% to 150% extra capability in the PSU, otherwise you will be running the PSU at its limits.

9. After this point you need a tech with a voltmeter, at minimum.

Point 2 above will not work unless you tell the PSU to start. This is usually done by the mainboard but can be done manually as well.
See http://www.youtube.com/watch?v=u0OKmIuNtmI for how to do so.