Hi shrimply.
I wonder if a software application is doing this. Things like this can often times be difficult to figure out.
I'm on Windows 7 at the moment but it should be similar to Vista. Can you check Event Viewer for some clues?
Control Panel>Adminstrative Tools>Event Viewer>Windows Logs>Applications
One of the other logs or even Task Schedular might also give you a clue. The last log entries after a resume from hibernation might give a clue if this is the case.