I have had no problems at all,I have pretty much the same system as yours except I have the Core2 6600@3.5GHZ and my power supply is a Silverstone 850,motherboard is P5B-Deluxe.
I would switch out your power supply and see for sure,I built my exact computer for a friend as mine with this PS
http://www.newegg.com/Product/Produc...82E16817194010 and he had similar problems,he didnt want to spend to much for a PS but ended up getting the same PS I have and has had no problems.