PC Won't post with more than 1 stick of ram

Yesterday I was using my PC playing BF Bad Company 2. And all of a sudden it goes blue screen. I restart and use again. After about some 1-2 hours, I was doing browsing and talking in xfire and it goes BSOD again, and I believe the BSOD crashed, because it froze and it didn't do the crash dump (I actually wrote the whole crash code). Well anyways after that I turn it on again, but it won't post.

Things I tried.

Switching ram around the slots.
Reseting the BIOS.
Leaving the PC unconnected for half an hour.
Removing USB devices and LAN connection.

The bios had OC settings that have been stable since more than a month ago, and I do a lot of gaming and it has been very stable.

Things that happened in the last week.
Friday 5th I took my PC to my friend's beach house and it work fine. When I returned home on sunday it also work fine. I leave my PC on the whole night and went to sleep. When I woke up (Monday), the electrical company cut out our electricity and it was restored Thursday night. The PC work fine and I went to sleep, last friday (yesterday) I was in college taking a test and doing some stuff, then at night it happened what I described at the beginning.

I usually leave my PC on 24/7, usually doing restarts every 4-6 days.

Both ram sticks work if only one is on the motherboard. When I put both in any config, dual or single channel, it does not post.

BTW Im writing from this same PC.

Current Setup:

TF7025-M2 Ver. 5.0 (2 years 3 months old)
AMD AM2 X2 4100+ ( 2 years 3 months old)
PDC22G6400ELK DDR2 800MHz (2 years 3 months old)
EVGA 8800GT 512MB (1 year 10 months old)
PC Power & Cooling Silencer 470W (2 years 3 months old)
X-Fi Xtreme Music ( 3 years 11 months old)

WD Caviar Black WD6401AALS 640GB (5 months old)
Antec P180 (5 months old)

Windows 7 Ultimate 64bit RC 7100

I have yet to try putting another set of ram and I cannot think of anything else at the momment.

DRAM timings manually set to 4-4-4-12-2T and 2.1V? Have you either tried bumping the CAS to 5, loosening any of the other timings to 4-5-5-12 or 4-5-5-15, or increasing DRAM / NB voltage?

Thanks for the reply. The ram is at default settings in the motherboard. Normally Ram is at 1.9v when OC is at 2.1. And this ram is default at 5-5-5-12.

Tried another set of Ram G. Skill. 1 ram worked. 2-4 sticks don't. Tried 1 stick in each of the different slots and they all work. So in conclusion any ram will work as long as is just 1 stick in the mobo. BUT when I tried 2 sticks 1 in slot 1 and one in slot 4 it posted, then at the "Windows is Starting" screen it hang. After a reboot it hang there again. After another reboot it hang in the bios logo screen and after another reboot it didn't post. I returned the GSkill to its respective PC and it work fine after 5-6 tries. In my PC whoever the Patriot pair won't work, unless just one stick is on the mobo.

Ok, yesterday very randomly after using the PC for a while with one stick of ram. I restarted, put the other stick and it booted normally. Was using it for a while, I could even play games, and decided to OC like it was before. I put the OC and it booted but after coming to the desktop it restart itself. I wasn't in the mood to deal with it again and went to sleep. So... after some 12 hours, I turn it on and it's working fine so far. OC'd. I checked the voltages with Everest and everything seems in normal ranges. Now that I noticed the video card was OC'd, and I remember I did that when I went to my friend's house last weekend. I now downclock to default clocks. I'll report any new findings.