Nvidia Geforce 8500GT graphics card buggy graphics then crashing computer.?

I was given a pc with a Nvidia Gforce 8500GT graphics card and it had worked for over a year. come to find out ( after having to do a clean install of the ENTIRE pc) it wasn't a stock graphics card. Ive gotten it to work with the newest drivers...but it is starting to bug out/crash on me. Any help on what I can do fix this problem?