Games acting abnormaly

i have a gigabyte motherboard with dual core 2.8 ghz and sparkle 9400gt .
for about 2-3 months i am facing problems while playing new games like bad company 2 , assassins creed 2 . while my freinds with ame conf. are playing the games without a glitch in my computer the fps is very slow and sound is shifting. but games like mass effect 2 , mirrors edge run mostly fine. one thing to mention here that my cpu temp increases often about 65-70c and the fan causes sound . can you please tell me where may be the problem and how to solve it