Very peculiar memory issue

Bizzmark

I'm at my wit's end on this. Bear with me while I explain my issue.. first, system specs:

Soyo SY-P4VDA
Pentium 4 1.6A
2 x Mushkin 256MB DDR266
1 x Corsair VS512MB266
550-watt power supply
Asus NVIDIA 5900 Ultra (with VIVO)
SB Audigy 2 ZS
Windows XP Pro SP1

Alright, for the longest time I had just 512MB of memory with these cheap (but totally reliable) Mushkin modules. Some months ago my brother wanted a new system built, and for a few days I had a 1 GB kit of matched dual channel Corsair DDR400 sticks available, before the rest of the parts for my brother's system came in. So I stuck them in my system - with ~1.25GB of RAM, everything was quite stable and ran much faster. Even EverQuest 2 was playable. : Then I had to put together his system and was back at 512MB, barely adequate for XP and most modern gaming.
So I decided to purchase a single 512MB stick of RAM and, of course, I went with Corsair. Not only did I try pretty much the exact same model in my system before, but I know the brand name and wouldn't buy anything else these days. It was delivered just yesterday, and I have ravaged my mind in trying to figure out exactly what's happening to my system and why. Here's what's going on:
With 1 GB total RAM installed (all 3 slots, 1xCorsair 512 and 2x256), I get massive data corruption in Windows. Most of the time I can barely open IE, Outlook Express, ICQ, Yahoo, whatever without getting a DLL error or, sometimes, Windows resetting my system completely (and that's with 'automatically restart' turned OFF under advanced system settings!!). BSOD is occasional, too. About 20% of the time I can get Windows to start up fine, do a bit of chatting, check my mail, but the test I've been using to try different settings has been running and playing a DVD - Windows will ALWAYS screw up if I try to watch a DVD with 1 GB of RAM.
The weird part: with any module configuration -besides- all 3 slots taken up and 1 GB total, Windows is completely fine and stable. 768, 512 or 256 MB of RAM in *any* configuration, any combination of 1-2 sticks in any of the 3 slots, it works perfectly. I have tested this extensively and it leads me to conclude that none of my RAM is bad. I have tried changing AGP Aperture size from 8 to 256. I have tried manually setting RAM timings (even though they're set to SPD by default, and all 3 sticks have exactly the same SPD timings). I have tried underclocking the RAM. I have tried uninstalling and reinstalling my VIVO bar drivers. I have tried 4 different versions of NVIDIA drivers. I know I could just stick with 768 MB of RAM, but I *paid* for 1 GB and I intend to use it. I will not put any more money into this system as I intend to buy an entirely new one and use this as an auxilary. There is absolutely no reason I've been able to find anywhere that tells me why exactly 1 GB of RAM would ever cause massive problems like this. This is the most mind-bending issue I have ever encountered in over 10 years of PC troubleshooting.

I'm willing to try pretty much anything anyone will suggest, any settings, just to get this to work. Why would 1.25 GB in all slots work but not 1GB? Please help.

If you'd like to read my motherboard manual (it's not very descriptive), go to http://dl.soyo.com/soyousa/manuals/p4/mp4vda10.pdf

soloz2

Have you tried to use memtest? I would suggest running memtest on each stick of ram independently. Then try it with the different combinations. there may be a problem with your motherboard when filling all 3 slots. As a general rule you should always stick with the same make and model memory. there may be an issue with your combination. Also, try your three sticks of memory in your brother's system. see if the memory works together there. if it does then it is probably a motherboard issue. I guess that's all I can think of right now.