ASUS A7N8X-X 32bit mobo. Athlon XP 3200+ 32bit CPU. All jumpers on the mobo are set to default. 3 sticks of Crucial 198 MHz (DDR-397) I had been having problems with unstability. While loading Linux Mint Mate 18.1, and 17.1 (both 32bit) the computer would restart in the middle of the install. I eliminated everything except CPU, RAM, video card and optical drive. Ran memtest 86+ 5.01. Immediately got fail with 3 sticks installed. I then tested each stick by itself. No failure. I then tried 1 stick in one slot, until I tested all three slots. All passed. I next tested 2 sticks, one in #1 Slot, one in #2 slot. that passed. I next tested 1 stick in #1 and 1 in #3. That failed. Placed one stick in #3 - pass. 1 stick in #2 and 1 in #3 - fail. Looks like anytime I combine slot #3 with any other slot or slots I get failure.

I have the ASUS User Guide. Says 3 1GB sticks should work. RAM is running in single channel mode - default setting.

Sorry, I am unclear on what exactly you are asking?

First thing comes to mind after reading your post:
Can you install the OS with just one stick of RAM? Once OS install is complete; install the other two sticks?

Not trying to install the OS. Why is memtest indicating a memory failure when slot #3 is combined with any other slot. All sticks pass memtest individually. All slots pass when used individually. The OS is wonky if installed as you suggest and then all three sticks are used. All sticks are identical.

Hardware breaks eventually.
So you've isolated the likely problem - if you can't fix it, avoid it. Simple.

I have not isolated the problem. 2 things come to mind. The chipset controlling RAM could be bad, or RAM is running in single channel mode instead of some other mode. Don't know how to run it in some other mode. What is single channel mode, and what are other modes?

Manual
bad board, just get it or had it?

Had the board since 2003. Waiting for ASUS chat reply. Don't know if they recommend Crucial. Crucial is about the best you can buy. The sticks each pass memtest separately.

The way I remember it, Socket A boards with 3 RAM slots were very persnickity beasts that had to clock RAM down to 166 instead of the otherwise expected 200 if using all three RAM slots. I don't remember ever keeping 3 sticks in one that had 3 slots, favoring the superior dual channel RAM throughput to having 50% more RAM. I still have one like yours, with 2 512M sticks, except made by Biostar instead of ASUS, and with a 2800+ Sempron instead of 3200+ Athlon XP: same physically twisted northbridge, same 6 slots, same NForce chipset, same Realtek NIC.

None of the Socket A boards support CPUs that feature SSE2, so mine rarely gets used any more. If I had 3 alike 1G DDR400s I'd stick them in to see if it even POSTs.

The mobo has no problem posting w/3 sticks. The OS will not boot, live session will not boot, or if they do boot they become unstable. I am trying it now with 2-1 GB DDR 400 sticks. Live session runs well but install is iffy so far. I started out with this mobo, Athlon XP 2000, 250MB of RAM running 98SE. Thought that was the bees knees after 95B on 486 machine w/32MB of RAM. Actually 98SE ran much better than XP on a Dell at work. Really would like to keep the first machine I built running for grins, and old SCSI large format Scanner.