I've seen this behavior with some of the 604e era machines. Mostly hanging on startup unless a monitor is hooked up, and/or on. I have experienced the failure of a machine to "wake up" the video correctly from time to time, after them machine has been deeply asleep. That is, not just monitor powered down, but hd spun down and all that. Only solution I ever found was switching the monitor for a different (newer) one. That seemed to have cleared it up. I'm sure there are other less drastic solutions.