My computer runs games slowly all of a sudden.?

I play games like Battlefield 2 , Cabal, and runescape(high detail). Then about 2 days ago, my computer started making thees game very skippy. or laggy. I have updated my graphics card. and i am defraging my computer. My computer is a HP Pavilion a6430f PC. I have also noticed that it has been shutting off randomly, and freezing. If any one knows anything that might help me solve this it would really help. Thanks- Ryan