Computer reboots automatically

Hi,
I have a P4 HT 3GHz. My computer reboots when i do some heavy activity. The CPU temperature goes to about 60?. I have applied Artic Silver on the CPU, but after few hours of work, the paste moves to the sides leaving very little of the paste on the surface. With a fresh application of the paste, the temperature goes to 48?. But slowly climbs to 60?.

Please help.
Rohit