Heeeeaaaaatsssiiinnkkkkkk.
This happened to me before. My heatsink was loose, and, anytime someone made a movement near the computer, the BIOS would screen and for the longest time I wondered why my machine was a seismograph. Apparently, one of the plastic pins wasn't able to stick in stably and was responsible for wiggling. All machines do this, at least the modern ones. If the processor is uncovered, the machine reboot-reboot-reboots.
I strongly recommend turning the computer on its side, or taking out the motherboard and laying it on some foamy/nonstaticy stuffs... or both, even better. Test out the stability there, then try resetting the heatsink. Otherwise, go try a new heatsink. If you're unfortunate, your motherboard in particular is too sensitive to heatsinks and is responsible for your troubles. My old ECS hated all three of my heatsinks. The ASUS is very good about them, nonetheless. :cheesy: