This I checked too. high performance, not disable monitor\drive.Just a shot in the dark: have you disabled all the power-savings / hybernate / whatever INSIDE the vm? Set to "power hungry/high performance" profile and fine tune sub-settings that can cause anything to go in sleep mode.
This problem only on 2008 server (not r2) and i'm think - maybe problem in Windows Backup? Today for test i disable backup.