[VIA] Via K7-266 Pro2 - System crashes if use more than 512 MB of memory

Question

Cannot use more than 512 MB of memory or system crashes. After running without a problem for over 2 years with 1 GB of memory installed, system started intermittent crashing, generally after 5 to 30 minutes of use.  I worked with the system manufacturer's tech support and they determined that it must be a motherboard problem.  I am hoping to find another fix rather than replacing motherboard.  System details:
MB: Via K7-266 Pro2 (MS 6380)
OS: MS Windows XP Update 1
Bios: V3.6
I've used Norton Systemworks and Norton Anti-Virus to keep system clean, and SpyBot and Adaware to clean adware
During troubleshooting, we found that it will run with 512 MB of memory without problems, but not with the 1 GB I had installed.  My memory was 2 x 256 MB installed with the machine and 512 MB I purchased from Crucial and installed.  Computer ran over 2 years with all memory installed without a problem.  After the problem started and during troubleshooting, we found that any combination of 512 MB worked fine (either the 2 x 256 MB or the one 500MB chip), apparently in any arrangement of slots.  But as soon as I went to higher memory, I was subject intermittent crashes.
Could not ID the source of the crash.  It sometimes happened when I was working with a single program open (various programs from IE to Excel to playing at GameSpy), had multiple programs open in background, or even when computer was sitting idle with only the startup programs active.
During troubleshooting we also did a complete re-install of Windows XP, updated the bios to V3.6, and checked bios settings, but the problems persists. 
What else could be the problem?

Answer 1

I am beginning to suspect a PSU problem.  I neglected to mention in my first post that when the system crashes, it does not freeze but has intermittent reboots.
I ran the memtest86+ test first on the two old sticks of memory then on just the one new 512 MB stick; both passed the test.  I also used the Prime95 stress test on both sets, and both ran without incident.
I used PC Alert to report CPU and case temperatures during the stress test.  The results maxed at 54 and 41 degrees respectively during the first stress test and 46 amd 39 degrees during the second test (I think part of the difference was due to a lower ambient, room temperature during the second test.)  The voltages reported during both tests were similar and were: Vcore1: 1.71v;  Vcore2: 1.26v; +5v: 4.76v; 3.3v: 3.31v; +12v: 11.60v; and -12v: 12.60v.  I am not sure if that information is meaningful, PCAlert showed all measurements as within the required range.
I periodically have cleaned the dust out of my computer, so I don't think that is the problem.
I used 2 power calculators with major differences in the reported status of the system. The test from pcpowercooling reported minimum and maximum requirements for my system (with the full 1 GB of memory installed) of 300w and 510w.  The alternate I found at jscustompcs (from a recent article about power supplies at Extreme Tech) reported a minimuum of 256w (no maximum is calculated with their tool.  The two tools had significant differences for memory, CPU, and PCI cards.  Off particular relevance to my problem was the difference in memory requirements; Pcpowercooling calculated memory requirements at 8w per 128MGB while jcustompcs used 12.5 per stick, resulting in different totals of 64w vs. 21w.  Which is right?
One item that makes me think it is not the PSU, however, is that the problem would occur with the 1 GB of memory while the system was idle, thus I was not using any of the hardware devices.  However, I never experienced the problem while using a device (e.g. DVD, CDRW, or tape drive) - either with 500MB or 1GB of memory.
My power unit is a Power Man #FSP 300-60BTV with 300w output, including +3.3v: 28amps; +5v: 30.0 amps; +12v: 15 amps; +5vsb: 20amps; -5v: 0.3amps; -12v: 0.8amps.  It also notes max output of +3.3v and +5v of 200amps.
My system also includes a Leadtek nVidia Geforce 3 Ti400 graphics card, DVD, CDRW, HP 14 GB tape drive, 1 case fan, and 4 PCI cards (modem, network, USB2.0, and Audigy 1).  The USB card is connected only to a bus in my Dell monitor and I have not used other USB devices.
Any thoughts or suggestions?  Do I need to replace the PSU?