Computer Freezing On Daily Basis

I've been having problems with my computer the past few months even after fresh install of Windows 7. The parts are almost all completely new maybe a little over a year and a half old.

I did underclock the processor in the bios in the past and randomly turn down the voltage settings trying to make my computer quiet as possible i dont know if this could cause damage. ive reset the bios since then.

I do a lot of video rendering and during the render a lot of times the computer will come to a crawl and the rendering will slow down and then speed up after 10 to 15 seconds even if im not touching the computer.

usually after some rendering but also just by leaving the computer on for a few hours sometimes (at least a few times a day) the computer will lock up. the mouse will freeze on screen then a few seconds later start to lag around the screen. sometimes at this point it will completely freeze, or graphics card driver will stop responding sometimes recovering sometimes not. sometimes it will stop lagging but the mouse graphics will be messed up. itll be oversized and glitched with lines going across it. hard to explain.

It's becoming very annoying. i dont know whats causing it. im assuming my hardware is damaged.