Hopefully by now you've figured it out. In case you haven't it sounds like a monitor issue. Due to the age of the machine (10 years), i would say you have a couple of possibilities, one is cold solder joints on the monitor pcb and the other would be a leaky electrolytic cap. There is also one other possibility, the cpu boards on these have multiple socketed ics. You can try pushing on them or pulling them out and reseating them.
Hope this helps.