K7N420 Pro -- RAM issues -- won't boot/crashes

Question

System info:
CPU: AMD Athlon XP 1900+
Motherboard Model: K7N420 Pro
Memory: (Varying, this is the problem, explained in post: but to begin with the system had 2 modules of 256MB PC2100 266mHz. Brand name unknown, some generic manufacturer. For reference's sake, the motherboard has THREE memory slots)
Video card: NVIDIA GeForce2 Integrated GPU, GeForce2 Integrated GPU, 32MB
Hard Drive: Western Digital 80GB
Other cards: None that I know of
Operating System: Windows XP Professional Version 2002 Service Pack 2
Power Source: Addtronics 300w (3.3v/18a, 5v/30a, 12v/9a)
Okay, so obviously we've got an old, old beast here.... But she's served me well, and I'd like to get a little more out of her. So here's the ongoing (very long, I'm sorry) saga:
The computer started doing crashing on me some time ago; lots of Blue Screens of Death, no error messages that gave me anything reliable to use for diagnostic purposes. Eventually it just wouldn't even manage to boot, each time would give me a BSOD. I opened the case up, checked for anything obviously visually wrong (blown capacitors etc), did a good thorough aircan dusting, popped out the two 256MB chips (let us call them Chip and Dale), and popped them back in.
This did indeed have a result- now we didn't even make it to the BSOD! All I got was a repeated ominous beeping and nothing coming onto the monitor at all. I then tried to put in only one chip at a time. Results: Chip works in slots #1 and #2, but not #3; Dale works not at all. (By "works" I mean the computer boots normally and runs normally, for at least an hour, the most I've tested it. Runs SLOW, of course, at 256MB, but runs. By "not works" I mean we get nothing on-screen, and the repeated ominous beeping.)
I presumed I had bad memory on my hands with Dale and that I needed new memory anyways, so I headed to Fry's and picked up two sticks of Patriot 512MB PC2100 DDR 266mHz-- the same specs as the old memory, excepting that it is 512MB instead of the 256. (According to Crucial.com, my motherboard's slots should each be able to support 512MB.) Let's call the Patriot 512 chips Bill and Ted.
Results with the new memory: Neither will work in slot #3. Logic dictates slot #3 is bad. Bill will work in slot #1 or slot #2. Ted will work only in slot #1. The computer will NOT boot if both Bill and Ted are occupying slots. (It also won't boot if Chip is occupying a slot and either Bill or Ted are in another slot.)
(Doesn't this sound like like one of those old logic problems where you have to sort out who owns which house?)
So the best I can get out of her right now is to stick Bill or Ted into slot #1 and proceed with the same 512MB that Chip and Dale used to give me back when they cooperated. However, new wrinkle: neither Bill nor Ted will get along with the computer for anything more than about a half-hour-- then, the computer crashes, tries to auto-reboot, and starts doing Black Screen and Ominous Beeping again. The RAM has to be popped out and put back in before it will boot normally.
*deep breath* I know that was long, thanks to anybody who read this far. Does anyone have any ideas as to what could be causing these problems and/or how to fix it? I've reached the end of all the knowledge I can put to work on this problem.
If I haven't been clear on anything, please let me know, I'll try to answer.

Answer 1

Hello,
Are you 100% sure that slot 3 is clean/dust free?
Memtest is a program that writes patterns to the memory.
The program then tries to recall the pattern(data) stored in the memory location.
If the memory is faulty the pattern(data) may have altered (goes corrupt) and the program will report an error.
Use just 1 stick at a time in slot 1.
Patriot Bill, let memtest run for say 3 hours.
If everything is ok you will not see any errors on the screen.
Then turn off and remove patriot Bill.
Insert patriot Ted into slot 1.
Run memtest again for 3 hours.
Again if everything is ok after the 3 hours it means that both Bill and Ted are ok.