it's either the CPU cooler, or you've overclocked it and the chip is unstable (i'm not understanding what you've done...)
did you raise the FSB above stock? or did you OC the chip to 2.8GHZ (if it's running 2.8GHZ, heat is definatly the problem)
also, if you OC'ed it, did you slightly raise the vCore
if you could provide some more information please...
replacing the CPU cooler is about all i can think of
I just recently built a new system using the AMD Sempron. I configured the BIOS to run at 2800(333) and moved the process speed to Fast. everyone else in my class that did this didn't have a problem. It doesn't work for me however. Whenever I power up, it works great for about 10min and then shuts down. heat is the only thing I can think of as a problem. I am attempting to replace the northbridge fan to see what happens..does anyone else have any suggestions.