Double-check that everything
is seated properly - your RAM, any cards etc. Its often a good idea to remove them and re-insert them to be sure. You said your temps are fine but are you monitoring them for the duration of your usage or just when you boot? It could be that your temps are rising after you have been using your system for a while.
HWMonitor is a good tool for keeping an eye on your temps:http://www.cpuid.com/hwmonitor.php
As suggested I would also try removing one stick of RAM if you have two and swapping. This will tell you if one of your RAM sticks are faulty. How long did you run MemTest for?