Memory problems

I've had a Gigabyte GA-8S655FX Ultra motherboard for around 2 years with 2x 400MHz 256MB memory modules running in dual channel perfectly. I recently bought another two 400MHz 256MB memory sticks bringing the total RAM to 1GB. But since I did that apps crash constantly and Windows quite often reboots by itself or gives me the blue screen of death. If I remove the two new memory sticks everything is fine. I formatted Windows and now it runs slightly better, but still with tons of problems. I tried using just the new sticks and removing the old ones and it also worked fine. It looked like the problem was only when all 4 memory sticks were in. I downloaded Microsoft Windows Memory Diagnostics which creates a bootable CD which runs 11 tests on the memory - and sure enough there were problems. I then started a series of tests to try to isolate the problem:

1. I tested each memory module individually, one by one. All passed.
2. Thinking it was maybe one of the memory slots on the motherboard, I tested one of the memory modules on each slot individually. All passed.
3. I tried 2 memory modules together. All tests passed.
4. I tried 3 memory modules together. All tests passed.
5. I tried 4 memory modules together. TWO TESTS FAILED!!!
6. I took another 2 x 400MHz 256MB memory modules from another computer, and tested them with both my 2 old sticks and 2 new sticks - BOTH FAILED!!!
7. I tried a 512MB stick with both my 2 old sticks and 2 new sticks - BOTH FAILED!!!

Basically, from these tests, I determined that it doesn't matter which memory modules I use, what their size is, or which memory slots on the motherboard I use - everything works fine until I put 1GB of memory in - then the ERAND and Stride6 tests fail and Windows works HORRIBLY. My motherboard was top of the range when I bought it - so I don't know why it wouldnt be able to handle 1GB RAM. The BIOS is picking up the memory and it's passing the boot up memory test. I have a 2.8 GHz Pentium 4 CPU (800 MHz FSB, 478 pin). '

Any ideas? Thanks
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SETPAuthor Commented:
According to the manual, the motherboard has:

— 4 184-pin DDR DIMM sockets
— Supports Dual channel DDR400/DDR333/
— Supports 128MB/256MB/512MB/1GB
— Supports up to 4GB DRAM (Max)

and it has these chipsets:

— SiS 655FX Host/Memory controller
— SiS 964 MuTIOL Media I/O

If for whatever reason I simply can't have 1GB RAM on this motherboard, what would be better - 512 MB running in dual channel or 768 MB running in single channel? I use Windows XP...
Have you tried setting the memory latencies manually, instead of by SPD?  If you have different rated RAM, the SPD settings may work for one set and not another.  Choose the slowest settings and work your way up.

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SETPAuthor Commented:
Thanks for the replies... I already have the latest BIOS. There is a newer BIOS but it is still in Beta and only updates the CPUID microcode.

I will try tweaking the memory latencies...
SETPAuthor Commented:
The BIOS only allows the following memory tweaks:

* I can increase the DRAM voltage by 0.1V only (tried it - didn't help)
* I can change the DRAM clock speed to any of these values:
     200, 240, 266, 300, 320, 333, 360, 400, 452, 482, 502, 536, 602 and 670 (it is currently on 400MHz)
* I can change the CPU clock speed, which adds one megahertz to the DRAM clock speed for each megahertz I increase the CPU clock speed by (this allows me to set the DRAM clock speed to values between the ones I listed above, but it also slows down the CPU)

I didn't see anywhere where I can change the latency...
I added 2 sticks of ram myself for a total of 4 sticks in my simlar system as yours and needed to do the following:

Try changing memory "command rate" from 1T to 2T, if there is an option in the BIOS memory timings. Many boards cannot handle the extra load of 4 sticks, and upping to 2T will cure it usually.
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