Repeated Freezes When Idle, No Blue-screen Or Mini-dump?

My computer locks up randomly and the screen turns black and unresponsive, at least once per day, forcing me to do a hard reset.There is no BSOD or Minidump created, although sometimes (maybe 75% of the time) there is a dump file created in the LiveKernelReports folder. The dump files in LiveKernelReports always point to atikmpag.sys. I tried uninstalling all video card drivers and using driver sweeper, and letting Windows use default graphics. I continued to get the same freezes, except no dump files were created at all in LiveKernelReports. I then reinstalled the ATI drivers, and the symptoms persist, now with dump files again pointing to the same atikmpag.sys.this only happens when the system is idle for a while. I never get freezes when actively using the system, but if I return to it after an hour or more, it's just a black screen and I must do a hard reset.I am running Win 7 professional 64-bit full version, 8 gb ram, 1gb ddr5 msi video card.Another strange symptom: After the hard reset, the computer has deleted all of my IE cookies. It also has forgotten the IE window size after these hard resets... very bizarre. In fact, if I make any changes to the desktop -- for example, by moving the icons or re-sorting them by name -- Windows doesn't remember this either. On reboot, it reverts to whatever I had on the desktop before making the changes. This does not happen on a normal restart/reboot. If I do not wait for it to crash, and instead I reboot normally, all of my settings/cookies/window sizes are preserved like you'd expect.

I've tried the following:

- ran memtest86, no errors after 9 passes

- ran seatools hard drive diagnostics, no errors found

- ran Hot CPU Tester 4, no errors found

- reinstalled/reseated graphics gard and memory sticks

- running with a single memory stick (same crashes occur with either memory stick)

- the windows memory and hard drive diagnostic tools, no errors found

- When running Driver Verifier, I get the same freezes, and still no minidump or bsod.

- Disabling all power saving options both in Windows and in the BIOS, like sleep mode, hibernate, etc.

- Updating all drivers that seemed out of date and upgrading bios to latest firmware.

Prime95 and furmark run without issue, and do not crash the system (presumably, because the system isn't idle when they are running). Idle temps (~30 degrees C) and temps under heavy load (about 60 degrees C) are well within the normal range.If I run in safe mode, it does not seem to occur, but I'm not positive because I've never stayed in safe mode for more than a day.