I have a recently upgraded my old computer that I built myself about a year ago. Not the best also not the worst.

Old specs:

AMD athlon 64 3200+
1gb corsair XM2
2x 320 gb Hd's
ATI radeon HD2600XT
ABIT UL8 mobo
500 watt ps
X-Fi audigy pro

New specs:

AMD X2 4200+
Corsaid XM2 2gb (2 sticks)
2x 320 gb Hd's (WD)
Ati radeon HD2600 XT
ABIT UL8 Mobo
500 watt ps
X-fi audigy pro

Now, my problem..... It over heats.
I installed my own heatsink.Which was $50, I do have the factory one.
It idles around 49C which I know is pretty high.
I play a lot of games.
When I go to play WoW, the temp starts rising to about 65C then over a period of time, say, 15 - 20 min, it will gradually work its way up to 80C. That's when my mobo
gives me the error sounds then restarts.
Now when I go to play a different game say crysis. Which takes around 4 times the processing power my temp goes to about 50C and idles.
Now, WTH is going on there. I don't get it I just want to be able to play my games normally.
My computer before I upgraded it wouldn't overheat when I went to play Any of my games. And I installed both of the processors myself. I checked for gaps and bubbles in the compound and I know how to lap. Still hasn't fixed anything. I haven't installed the factory processor because I wanted to make sure this one would work so I didn't have to waste $50 on a heatsink.

check the sink for a plastic cover on bottom, replace the thermal pad if any with arctic silver 5 after cleaning the sink and cpu. make sure to reseat the sink properly, clean the case and fans thoroughly, also make sure the sink supports your cpu

Did all that.
Who would leave the cover on?
3200+ and X2 4200+ are both 939
I just installed the factory sink now and going to try the games.
Everest, speedfan, and Hardware monitor are all giving me around 39C. So im hoping it will go up around 10C and stay there while I play.

I am having a similar problem. I upgraded at Xmas from a 3500 single core to a 4200 X2. (939). Difference is, I have a 530watt hiper power supply (among other things).
Full Spec:

Mobo: Asus A8R-MVP Crossfire
CPU: Athlon X2 4200
RAM: DDR3200 1gb (2x512 geil) 2gb (2x1gb) generic brand. Miss-matched is bad I know, but it ran beautifully on my 3500.
Cooler: Arctic Cooling 64 Freezer Pro.
Heat Transfer Goo Stuff: AC MX2
Graphics: 6600GT (again, not out of choice, everything bar the extra gig of geil ram, the CPU and the Mobo (and the extra soundcard) came to me from a friend for free).
Couple of 120mm fans in the large coolermaster case (case is cleaned every couple of months).

Average temp, as I have quite a warm house, is 38-42 idle. originally this only raised to 45 at most when I was doing the most complex work in illustrator or an intensive part in a game (tried sitting in the middle of an arty strike in bf2 on full graphics...) and I never had a stutter, but recently I re-installed guildwars (after a long period off after upgrade to X64 XP), and now I can barely use my PC. As we speak, I have 7 windows in opera, OE open, and MSN (weak stuff...barely call that a load), and yet Asusprobe is reading 49/50 degrees C.
The processor has been overclocked, a month after original install, with no problems, and was at a stable ~2.4 ghz (used the automatic % overclock option on my bios, I was lazy this time!), and it's been fine ever since...until last week.
Any explanations? I'm not so much interested in cooling it down...as to why the flaming hell my processor is reaching these ungodly temperatures in the first place...only difference being mine has recoreded as high as 87 before system emergency shutdown
and the temperature can double within 5mins.
I know how to seat the heatsink properly, having had it on and off enough times, and I know how to spread the gunk correctly (bad word to use for it I know, but I'm tired after a 12 hr shift at my local pub yesterday, so being lazy), no bubbles or anything, yet I can't find any reason to explain why in the blue f..... my CPU is doing this?!
Any help appreciated.