I have found that with higher overclocked speeds, like 768MHz, my phone reboots because it becomes unstable, at least thats what i'm assuming happens, my sense also restarts if i have it overclocked to high, try the next lower max speed that SetCPU will allow and see if it goes away. i haven't had a problem with it since i went down to 748MHz
My Eris also does not like being overclocked and resets, try lower speed or default. It could also be an app doing it. I was using Fring for a while and found out it was resetting my phone twice a day for some reason.