I would not go through the trouble of reinstalling your OS. Like the fellas said above, check your power supply. If I were you, go into your bios and/or video card settings and find out if your drivers are compatible and are up to date. If all is good, then double check your resolutions on your monitor. If thats good, then perhaps your video card is overheating, or some other peice of hardware is overheating.
If you tried just about everything, and nothing changes, I have an advice for you: Dont use the Hi Res texture pack! It would save you a lot of time and headaches.