Sounds like your computer is overheating and in particular your video card, when was the last time you cleaned out the inside to remove any heat trapping dust?
Unfortunately the 9500 GT is not up to gaming and is more suited to office and media type use, read the review below;
Couple of things for you to do, 1st download and run Speedfan http://www.almico.com/sfdownload.php
post a couple of screenshots with your next reply, one before any gaming and one after a couple of minutes gaming, this will allow us to check your temps and voltages then download and run gpuz from here http://download.cnet...4-10788513.html
this will check out the performance of your video card, let us know the results.