During startup it restarts just before the User Profile login screen should show up. (I can still access the BIOS) It does this with both Ubuntu 10.04 and Windows 7 (even in safe mode and startup repair). Sometimes it will boot up normally, but not every time. This problem started after I installed a new heatsink on my CPU. The computer would not start and I realized that the RAM was partially unplugged. After I put it in the PC ran fine for a day and then this problem started. I am thinking this may be a problem with the RAM or Power supply. Any ways to test for bad connection or bad sectors and any other ideas as to the root of the problem are appricated. Detailed specs below.
PSU: Eagle 650 Watt
RAM: Corsair 2GB DDR2 (Vintage 2007)
GPU: ATi Radeon 4850
CPU: AMD Athlon 64 X2 5200+
MoBo: ASUS M2N-E SLI (That is an nforce 4 chipset)
SND: Creative X-Treme Gamer Titanium
OS1: Windows 7 Home Premium 64Bit
OS2: Ubuntu 10.04 32Bit
Download Memtest86+ and run a scan, that will tell you if your memory is stable or not.
As for your power supply, that's a bit trickier. If you have a multimeter/voltmeter, you can check the voltages on the 3.3, 5, and 12 volt rails (Orange, Red, and Yellow respectively) to make sure they're at the appropriate levels.