I'd like to start off by saying I am not overclocking anything (yet

) It's all stock speeds. But this seemed to be the most relevant place to ask.
I just built a new rig this week and with only 2GB installed, it's completely stable. But as soon as I add the second 2GB stick, the PC starts to intermittently freeze. Especially when running memory intensive progs like games. (Far Cry 2, Stalker CS) Yet I have no freezing with only 2GB in and out of games. (Stalker still crashes, but that's just a buggy game

) I don't beleive it's OS specific as this happens in both Vista x64 and W7 6801 x86.
Sometimes it will run for hours problem free when just doing 'desktop' tasks, but as soon as I try to run a game, the PC freezes completely, requiring a hard reset. Sometimes it happens after after a few minutes, other times straight away.
Specs:
ASUS P5Q PRO mobo
Jetram JM4GDDR2-8K Dual Channel Kit
Welcome to Transcend Website
(rest of the details are in My Specs)
The BIOS memory timing is set to auto. For the DRAM voltage I have tried using auto, 1.8 and 1.82 and it still freezes. (I think auto is 1.8 anyway)
I have run memtest on both sticks individually, one with 20 passes and the other about 8. Both sticks showed no errors. However when I tried memtest with both sticks installed, it froze completely at just 28% on the first pass.
I have now googled my self silly reading up on this, to the point that nothing is sinking in atm

The problem for me is that I'm still not familiar enough with which BIOS settings to alter / test without risking damage. Since a lot of you have a shedload more h'ware OC'ing experience, I'm hoping you can help point me in the right direction