Ram only fails when both are in

My first build is giving me serious trouble. I got it working for about 6 months with win7 32bit, then started getting increasing amounts of blue screens, which consisted of Bad_Pool_Header and IRQL_Less_Than_Or_Equal_To. I never got 64x working on my computer but god knows how ive tried.

I've tried reinstalling windows but it wont get through installation for either a 64x or 32x OS. It either blue screens during extraction or after first restart, saying windows wasn't installed properly.

kindly people suggested that i use memtest86 , which i have. I tested both sticks individually and they come out fine, but when i tested it with both in slots 1 and 3, i got lots of errors. The first run found 8 errors, the second found 3 and the third found 7. A little guidance would be appreciated.

TLDR: when i run memtest86 with two sticks of ram i get a different ammount of errors each time, and both sticks come out clean individually. What does that mean?

Thanks in advance

ASUS P5Q Pro
Intel Core 2 Duo 3.0
G-Skill 2x2GB DDR2 800 (5-5-5-15)
BFG 550W
Western Digital 500GB (only about 460 show up with thats another issue)
HIS 1GB DDR5 9860 graphics card