Cold boot memory errors

Hello,

This just started to make me go crazy  

The system just hangs on memtest (or windows is starting...) if left off over night. But after few restarts it works fine! Think it's the memory but didn't have time to try it out only with one pair at a time. Any help...

Tests:

Memtest after PC was left off over night = FAIL, millions of errors (BSOD at windows is starting...)

Memtest after reseating ram modules = PASS, no errors

Memtest after few restarts = PASS, no errors

Memtest after 12h of use / restart = PASS, no errors

Prime95 stable over night...

Memtest, tryed every module in every slot with 0 errors afte PC was warmed up.

System:

Q6600 (stock, 6 yrs old)

ASUS P5QL-E (4 yrs old)

2 x 2GB Kingston DDR2 800 (1,5 yrs old) + 2 x 1GB TeamGroup DDR2 800 (5 yrs old) both pairs the same specs (5-5-5-18, 1.8V)

nVidia GTX 560Ti

PSU 850W (replaced, didn't fix the problem)

OS not important (happens in windows 7, 8 and Ubuntu)