If Server 2008 doesn't have the same reboot, it sounds like it's probably a driver incompatibility.
How did you think to disable Enhanced Speed Step? Was that a suggestion from the Hyper-V side?
Just an update for anyone who might be reading this: I misspoke about the MB version, it is the S3000SH, which is not compatible with Hyper-V. Any time I tried to start up Hyper-V the system would bluescreen. Thus I just moved to using VMWare Server and now running 2xRedHat EL5 and 1xSunOS VM without any issue. I also haven't been able to refind the reference to the speedstep setting.