Member Rara Avis
Actually, I saw it when you originally posted, Jamie, but was hoping someone else would answer. Because I only have two suggestions, and you ain't gonna like one of them.
First, your Windows setting for Power Management may be getting overridden by the bios settings of the computer. When you turn on the machine, there is often a message that flashes by telling you how to get to the bios setup; holding down the Del key is common, sometimes Ctrl-Alt-Esc is used. Once you figure out how to get there, make sure the bios setting match your Windows settings.
Second, and this is the one you won't like, one of the classrooms where I teach has been experiencing a very similar problem. They got new P3-800 Compaqs about three months ago, and ever since then the computers all go into hibernation during lunch break. Worse, they refuse to awaken on command, forcing a time-consuming reboot. Their dealer apparently tracked down the problem, though it wasn't resolved when last I taught.
All the machines need new motherboards. Ouch.
I don't teach again until somewhere around Aug 8, but I'll see if I can find out more for you. Maybe the dealer is aware of a tech report from Compaq or some such that better explains your problem and subsequent options.