your psu should be plenty and a good brand to power your system, but it "could" have problems and be defective.
what we can do as a check is this:
download and install sensorview pro (free 30 days) that will enable you to keep an eye on voltages and temps as we work the system.
then with sensor view installing and running
begin running a spyware scanner, anti virus scan, defrag your hard drive & play MP3 music all at the same time
then record on paper what your voltages and temps are after 5, 10,15,20 minutes of this max workout
the real goal here is to see if you can record the voltages and temps when it does a restart
also: when is the last time you did a refresh of the CPU thermal grease ??
is your CPU heat sink free of dust ??