Computer reboots once it hits 63C

This problem started a while ago, once my computer has been on for about half an hour it starts rebooting its self over and over again.
I thought it was a software problem, so I reformatted it, but no, the problem persists.
I opened up the case to make sure the fans were working and they are, but the heatsink is running a bit hot.
What can I do?

Model: Gateway GT5026E

Have you ever had any real problems with dust in the casing before?

Im assuming your CPU has not been overclocked.

If theres an overheating problem around your heatsink you could try opening up the case again and checking for dust clogged in and around the heatsink, fan, and CPU.