AS6920G freezes or shuts down

First of all; forgive me my spelling, and if I don't understand technical terms without asking for an explanation on their meaning

I have problems with an AS6920G. Intel core 2 duo T7500, 4 GB RAM and ATI HD3650 graphics.

Right now I've installed XP - apparently on the only harddrive that works in the computer, and it works just fine. But both with the original harddrive with the Vista that was pre-installed, and with pretty much any other harddrive with both Vista and Windows 7, the pc's nothing but rubbish :S The problems is the same en both Vista and Windows 7, and in both 32- and 64-bit. Mostly it freezes completely, and the only thing I can do, is to shut it down by pressing the power button and holding it 'till windows shuts down. Other times, the pc reboots without warning, not even closing windows first. There's no pattern in what it does when, or how long time it can run without going down.

According to CPU-Z, the CPU has a core speed at 2.2-2.4 GHz, but it's only a 2.2. I think the problems with freezing and shutting down startet (without being completely sure) after I installed Windows 8 Preview. Before that, the core speed was lower. The problems startet out as occasional errors, but got more and more frequent. Until it rarely worked for more than max 15 minutes at the time.

I've tried a number of things:
1. Changed harddrive (many times) and installed different systems (Vista, Windows 7, Linux Ubuntu and XP) -- Only XP seemes to work, and only on one harddrive, which is only 160 GB. (It has a lower power consumption than the original harddrive, but higher than the 500GB WD I've also tried)
2. Changed the RAM -- when I did that, the computer worked most of a day on Windows 7 without problems, but after that, it was back to the same.
3. Reset BIOS to default settings -- without any improvement
4. Flashed BIOS to 1.16, Acer support told me to do that.
5. Have tried another AC adapter from an older Aspire
6. Cleaned the fan.
7. I've had the WiFi module disabled.

Please help! I'm running out of ideas, and would like to go back to either Vista or Windows 7. Anybody got a solution?

 

Try booting with one ram module, if that doesn't work try the other slot. If that doesn't work try the other module separately also.

Hope you can get it working... these aspire 6920g models are AWESOME even though they are old.

 

Can it be the RAM modules or slots when the computer is running without brakedowns on XP?

We've thought of trying to take out the battery on the motherboard tonight, and put it in again in the morning, and install Vista from recovery disks, so we have a fresh install.