Couple of problem with my computer

My Specs:
Intel® Core™2 Duo
ASUS P5N-D Motherboard
NVIDIA Geforce 8500 GT
2GB Ram
With Windows® XP Professional installed on C Drive

I have OC'ed my computer about a month ago and it was working smoothly with no problems at all. I know this because I had installed many things after that.

My problem is that while playing games the screen goes black for a second and then comes back. And after that, the textures keep on going from bad to worst. It happens unevenly like sometimes when I just started the game, and sometimes about after an hour.

I think this would be my card warming up, but then again, it's running in good condition right now. It mostly happens with older games.

Recently I have experienced my screenshots don't work, my windows would stay even after I minimized them, bigger pictures won't load, etc. I need to restart to get back again and for 2-3 hours my problem is solved but it comes back after 3th or 4th hour.

My computer even freezes entirely for 10 - 15 seconds sometimes.
Please help me on this!

IMO you might had overclocked to far and corrupted your Windows installation. You could possibly have memory issues as well. Try to restore to the default setting and see what happens.

Probably you are having some hardware issues elsewhere and not GPU problems.

Run the game in windowed mode rather than fullscreen. It usually works in most of the cases.

And for screenshot not working you can try the old-fashioned method viz Open MS Paint and press CTRL+V to paste the copied screenshot, then save the image as a .JPG.