I have a 5 year old computer which has been behaving strange recently, such as:

- Graphics card occasionally not being detected at boot-time and falling back to onboard graphics.

- CD-ROM drive failing POST test.

- DVD+R drive can only read CDs, not DVDs, no matter if they're burnt or original pressed ones.

- Flickering monitor (that could just be the monitor itself dying).

- Shutting off by itself (I've never witnessed it actually happening. When I leave a download going overnight I find it off in the morning).

- Sometimes when I use the mouse the computer freezes, then suddenly the cursor starts moving erratically, clicking everywhere. The computer's speaker also beeps as well.

Problems occur on both Linux and Windows.

I'm wondering if my power supply needs replacing.

The specs are:
P4 1.6 GHz
256 MB RAM
ASUS P4S333VM Motherboard
40 GB Seagate (Primary Master)
80 GB Western Digital (Primary Slave)
Samsung CD-RW (Secondary Master)
HP DVD+RW (Secondary Slave)
300 W Codegen PSU (it's a horrible generic one, so I suspect that is the culprit)

Or maybe my whole motherboard's dying?

It could be failing, or your system could be overheating. Have you checked that the fans are working normally, and have you cleared away any dust from the interior of your cabinet?

Yes I've given the case a cleanup and reconnected the cables, etc.

The fans seem to be working okay.

I've experienced similar problems with my system in the past and it inevitably points to some hardware problem. In my case it was actually a fried CPU (AMD Athlon XP 2600 processor). It was a frustrating period of diagnosis and it could point to almost anything.

Unfortunately the only way to isolate a hardware problem is to take each component, replace it with a fresh component and then try again.

One clue is the graphics card though as you mentioned. Why don't you remove it and try the on-board graphics and see if you can reproduce the issue?

You can also refit it properly and try again later if that works.

I recently had to replace two PSUs
for various reasons
and I have to do another

One thing I did learn
is to get a good quality one
some are very optimistic in their wattage claims

If yours is a "300" watt PSU
on the label
it may be much less in reality
putting a strain on the whole system

and if there are any mains supply issues
like under/over voltage, spikes & surges
A cheap supply is going to give you problems

consider
you are driving 2 HDDs
2 CD drives
and you mention a video card as well
on top of the normal stuff
go for a 400 or 450 Watt PSU
and a good quality one


floppy

Yea its definately the psu. You

That's a lot more than neccessary, Hard drives don't use much power, and most of what they do use is at boot time when the drive is spinning up(when CPU usage is low), CD drives don't use much power either. Most video cards aren't nearly as bad as power guzzlers like the geforce fx5900 and the 20+ pipe cards out now(so it would be good to know what kind of card it is). A 350 watt fortron or a 330 watt seasonic should be more than enough. Maybe a "350 watt" or "400 watt" version of one of those overhyped "gamer" power supplies made by questionable manufacturers won't be enough though,

As to whether your motherboard's dieing, you could check for leaking capacitors on it.

Also, make sure to get a power supply that is appropriately balanced for your motherboard, ie. if your motherboard uses the +5 volt rail to power your CPU you'll probably need an old power supply (one that conforms to ATX 1.2 or 1.3 standards), if it uses the +12 volt rail, you should be fine with any good new PSU. If your motherboard has a 4 pin square connector with 2 black wires and 2 yellow wires, it uses the +12 line. if it has a 6 pin flat connector with 3 black wires and the other wires red and orange, it uses the +5 line.

It sounds very likely that your PSU may be the problem, based upon the combination of symptoms you describe. PSU quality tends to have a rather large impact on overall stability, but so does RAM.

One common scenario you'll see is that things like motherboard and CPU get blamed. It may be true that they're toast in those cases, but what is often overlooked is that it's often the result of having a sub-par quality PSU driving them over time. Low quality PSUs tend to have poor voltage regulation, over-voltage/current protection, and often have far-fetch specs in comparison to what they can truely sustain. This can often stress sensitive electronics and do things like shorten their life, subtly damage them (this is the worst thing since it's very hard to troubleshoot since the component(s) isn't outright failing, but has become unreliable and intermittent.), or even outright fry them. On the other hand, there's the aspect of the quality of the electronic devices in question. Some motherboards, for example, are more resilient and able to withstand more abuse (as in being fed by crappy PSUs) than others. I've seen some motherboards survive really well unders circumstances that would normally kill most. I have this one old Asus motherboard that takes a licking and keeps on ticking, and is rock-solid to this day, for example.

I would look at your situation like this:
If you have a crappy bargain PSU, get rid of it. If it turns out not to fix your problem, it's still good that you replaced it, with hopefully, a quality one, for the sake of the rest of the components that are still good in your system, as well as any future ones.

Thanks for all your responses so far.

I tried moving the monitor's power cable to the mains instead of running it through the power supply, and that seems to have stopped the flickering.

I'll get a new PSU soon and tell you the results. I don't want my hard-drives frying next.

Just to be safe, replace the mainboard battery (cell) as it must be almost finished after 5 years.