58C is pretty hot and if you're checking after gameplay it could have even cooled a bit until you checked it. So your CPU could easily be hitting 65 and possibly your shutoff temp is 65 degrees. Try reseating your heatsink and processor, sometimes if you seat it better it'll make better contact resulting in lower temps. Also I'd suggest possibly looking into an aftermarket heatsink and better fan.
Nubius, or anyone else, how accurate is the SiSoftware Sandra for reading temperatures?
I find that it works pretty good. On my MSI board the program that came with MSI Called CoreCenter would read the temps about 4 degrees higher than SiSoftware Sandra...BUT...SiSoftware Sandra was EXACT with my BIOS temp readings so I assumed SiSoftware Sandra was more precise, but went by the corecenter temps because it's better to assume your temps are higher and try to lower them than it is to assume your temps are low and fine.
I've also used the program on my ABit board which gives me the exact readings in the BIOS and with Motherboard Monitor 5 so IMO and experience I believe SiSoftware Sandra delivers VERY accurate temperatures. I have not yet seen it display a temp that was different from my BIOS so it's very good in my opinion.