The heatsink does not feel hot. Could be faulty readings, but I really don't know

My computer has been running VERY unstable since I built it. I have all the latest drivers installed, the newest BIOS for the mobo. I am lost as to why it is so unstable.
If I erase the lead off the L1 bridges, to bring it back to it's locked state, it seems to run a bit more stable. Yet, the temp readings are insane. I want to believe the readings, tho. Because after roughly 5 minutes of Q3'in, my computer resets. While I'm there, I go to the BIOS to check the temps. It showed my CPU at 140 degrees!:Q
I can't do much, because of the problem. Is it possible that it's a faulty CPU, or something? Windows loads fine. I can run mIRC, and ICQ with no problem. But if I'm staying busy browsing the web, my whole computer freezes (even ctrl-alt-del doesn't work), and then after roughly 10-15 seconds, it restarts. I have tried running it at 800 with the VCore upped to 1.80. Still nothing, which shouldn't be the problem, considering AMD says it only needs 1.70 (even tho my mobo says the defauly is 1.75).
Is it possible that it's a faulty RAM chip? If so, then why would it not make a difference, whether it runs at 100 MHz, 133, or even 138 MHz? Or whether it's CAS2 or CAS3 doesn't make a difference?
This is the first AMD system I've built, but the 5th system I've built. I am very stumped, and have no clues as to what I need to do.
I have taken the HSF off a few times, and there is a bit of thermal compound on the DIE itself, but nowhere else on the chip.
I'm guessing the HS is touching the die, cuz there is a bit of thermal compound on the HS where it contacts the die.
Any help/suggestions/comments of any kind would be great!
As I said, I am very stumped. The Tai-Soli HSF is supposed to be sufficient for up to 1 GHz, yet I can't get my system running stable at 800 MHz, and not even at 500 (which is what I'm running it at now)!
Thanks!