Could be memory, could be motherboard, could be lots of things. I would start by checking all the fans. CPU, PSU, GPU, any fan in the system. Make sure something isn't overheating. Could be an easy fix, before you start checking into other hardware.
I'm having the same problem lately. I thought it was a PS issue because I can hear the CPU fan slow down a bit as I "do" stuff, download, open new browser widows, start programs, and then it just restarts. I changed the PS and I'm still having the same problem. I cheched the CPU temp and it seems fine also. I think my memory might be going bad. I keep AVG/ZoneAlarm running and I'm pretty good about running AdAware and SpyBot pretty often so I don't think it is a software problem.