1.) if your CPU was overheating, then your system would just shut down, not restart. If the system is plain restarting (no blue screen, etc) then the problem is most likely your rubbish PSU. Chances are that thing doesn't produce 400W, and whatever current that it actually can provide will not be enough on the +12V rail which is where it's needed the most.
2.) those temps are too high for an Athlon64. the max operating temp for that CPU is 65C.
even with the stock fan that comes with it you should be running at about 25C~30C idle, and 30C~40C under load (make sure you use thermal paste/grease and not a cheap thermal pad)