CMX4GX3M2A1600C9 very bizarre behaviour

So I recently bought this new rig consisting of Lynnfield i5 750, P7P55D mobo and of course Corsair dual kit CMX4GX3M2A1600C9. Booted the system and it went fine, rebooted after installing some software and it went fine, rebooted and led on mobo lit up (meaning POST didn't succeed). Took out sticks and replaced them, same thing again. Switched off from power supply and rebooted and once more everything went fine, rebooted and all of a sudden BIOS + Windows recognise only 2gb out of the 4gb Anyways i continued with this rebooting/POST failing/POST succeeding but 2GB out of 4GB recognised/everything works fine! pattern for about three days up to the point of driving me mad The really bizarre thing is that there is no pattern at all in fact. Sometimes everything works fine and sometimes some other failure crops up but in a random fashion.

Today it passed my mind to take off the sticks once more and test them one by one. So I started with first stick, boot 1 went fine, boot 2 went fine, boot 3 with intel burn test went fine etc etc etc..... up to the point that I overclocked rig up to 3.8Ghz and it was perfectly stable and still is. After about 5hrs of testing took out stick one and replaced with stick 2. Boot 1 went fine, boot 2 started same problems again. So stick 2 is definitely the problem. The thing is that i ran MemTest on stick 2 for 4hours and it didn't find one single error, however i than rebooted and POST got stuck on the ram led once again i.e. back to square one

I think this is a really bizarre situation, I'm now really tired of messing around and would appreciate some help. I'm suspecting a defective stick but cross-fingers prob might be solved.

Well you haven't mentioned what voltages you've set. It's possible that the one chip is weak to initialize during a cold boot in which case I would definitly contact Corsair and get the chip replaced.