Computer rebooting itself! (First Build)

[ryguyryan]

This is my first build and I just finished the PC a few days ago and for some reason it keeps restarting. it happens more frequently when playing games but it does also happen when browsing the web and stuff (mostly watching videos). I have so far manually turned up the fan on my video card, replaced my video card with an old one that I had and replaced my PSU with an older one I had also, switching one of the pieces of hardware out seems to work for about an hour and then it starts to restart itself again. The temperatures aren't too bad either:

CPU is around 30C
GPU (with fan on full speed) is about 30 idle and 45-50 in game
MB is at 35

If anyone can help me out that would be great because it is fairly frustrating to have to deal with this, especially with my first build haha.

Thanks

system:
GPU: ATI Radeon 6870 (replaced with older Nvidia GeForce 7600 GT)
CPU: AMD Phenom II
PSU: Antec 750W
MB: ASUS M4A87TD EVO

 

[Emerald]

did you run a memory test yet?

 

[ryguyryan]

I did a memory test and nothing came up so I assume that there isnt anything wrong with my memory. My PSU was upside down tho and i flipped it back but still no help... i think it may be an issue with my GPU temp cuz if I turn the fan on faster it seems to work a bit longer but still shuts off

i found out that default fan speed of the gpu the temp is up to 60C and full speed is at 40C