Issues with random slowdowns in all games.

I just purchased a 7950 and replaced a GTX 570 that was then given to a friend. Now my previous card would also do this leaving me to believe its not the graphics card at all but possibly something else I didn't think to look at like maybe my power supply? I get random slowdowns during game play where I see my FPS go down in game. The FPS drops more dramatically the more GPU intensive the game is. I recently decided to have the catalyst running on my second monitor while I was playing a variety of games such as League of legends/Borderlands 2/Modern Warfare 3/World of Warcraft and while playing all of the games above I noticed my CPU usage/activity would go from around 50% usage to 1 or 0%. A lot of people say that's not the best indicator but it was perfectly in sync for when the FPS drop would occur. My CPU was also upgraded this year to a Intel i5 2500k not OC. Temps are fine and have no issues with that. Now i've also tried 4 different drivers after doing a driver sweep in safe mode and I have had the same result every time. I'm wondering if my Corsair 650 watt may not be strong enough or simply just getting old(little over 2 years old). Could really use any help I can get to know whether I need to send this card back in or not.