Intermittent Boot


I'm having a problem I've never experienced with a mac. I was listening to some mp3's when the
monitor image zoomed in then blacked out.

I though My monitor went. I replaced it with a
small monitor but that didn't work. I took out my ati raideon 9000 video card and re-installed original video card, it booted. then it was fine for a day then would not boot. Then I saw a large black dialog box with a message and a watermark of the power button, I have never seen that before. I then thought my video card went so I hooked up my previouly thought blown monitor
and it still didn't boot. put the smaller monitor on that wouldn't boot, then it booted, I selected a 9.1 O.S. to do some work on an application not OSX compatible, upon rebooting OSX it froze with pointer in left upper corner. hit reset button then it booted. now it would boot today.
max osx g4.

any thoughts thanks