Memory has been checked with memtest which reports no errors.
I have two issues, possibly related:
1. After it has been on for a while (3-7 days) applications tend to end up "not responding". They stay that way for 10-30 seconds, and then they run again. It doesn't run many apps to begin with, and I've closed one by one, but it still happens with windows itself after all have been closed. A reboot resets the "working period". No malware or trojans.
2. Sometimes, particularely under heavy load, it suddenly restarts. It's not a bluescreen because I've turned of automatically restart on those. The event log does not show anything of interest except that the reboot happened unexpectedly. I've checked the temperatures with core temp and under 100% load (using OCCT) they max out at 65-75c which should not be enough to trigger a reboot.
The reboot happens rarely, maybe once a week if it is under normal load. By running OCCT I can trigger one of these reboots in 1-5 hours. No other apps are running under this test.
Having speedstep enabled seems to make the reboots happen more often, but disabling it does not get rid of them completely.