I just upgraded my rig(939) with the 4800+ x2. I used the stock cooler as I don't intend to OC this any time soon. I didn't use any additional thermal compound, just the square that came on the bottom of the heatsink. I noticed that after about 10 minutes the PC shuts down cold as if the power was unplugged. No warning or anything like that. One of the first things I did was install the x2 driver for xp that I got from amd's support site. After it prompted me to reboot it shut down during the post. Not a windows thing. I tried to start it up again and it killed in about 30 seconds(during post again). I left the machine shut down for a few hours and when I started it up it ran idle properly for about 10 minutes, then shut down again. I think it may be an overheating problem, but I don't know why this is happening as I am not oc'ing this thing at all. I have 2 huge 80mm fans. I have a 550w power supply
All signs strongly point to temperature related shut down. Check your temps at startup in the BIOS or using a third party program. If they are high, stock 4800 X2 shouldnt idle over 45 I dont think, even in a warm house, reseat the heatsink and look on the chip to see if good contact is being made.
I agree sounds like heat. Get some thermal compound (any kind dosent really matter) but arctic silver 5 or what ever you have that is known not to be bad, clean off the proc and the heatsink with alcohol really good then smear the compound on and put it back on being sure that it seats properly and is not at any angle and push down the lever.
My aftermarket cooler was on so good on my 3200+ that the arctic silver created a vaccume or something to that effect that it pulled my processor out with the lever down, scared the hell out of me, but it was fine in the end, so make it that good and try it out.