Monitor won't wake up


I have a G4 (AGP) with a 17" Apple CRT monitor (ATY,Rage128pro) that sometimes (unpredictably) won't wake up from sleep, although the rest of the computer does. The screen simply remains dark. I then have to do a hard reset to get it going. Any ideas?
I get that sometimes on my PowerBook G4. It seems to do that when I've had it in Sleep mode for a long time or added an external peripheral whilst asleep and then tried to wake it (oops).
I also discovered this morning, when the problem re-occurred, that after waking up the computer and finding the monitor still asleep, I've also lost the USB connection of my keyboard and mouse to the computer via a powered hub. What is odd about this is that the USB must be fine in order for my keystroke to wake up the computer; it's only after the computer wakes up that I've lost the USB connection (as well as having a sleeping monitor...).
It seems like the sleeping monitor could be caused by the sudden loss of USB, although I have no idea what could cause this.