GTA IV artifact crashes. Please help?

It doesn't have a specified ammount of time in which I can play. It runs very well on my system. And I played it for a year with no crashes, but now I play a few minutes then... BAM ! Artifacts and crashes! Sometimes this messes up my system and graphics and I have to restart. Please, do you have any idea? I have a Geforce 8600 GT. I know it's old, but it ran well for almost an year.