Ok wait on the motherboard. The beige G3 is a strange bird, I have a bunch. All do the same thing. The issue is it tries to push to the newer monitor but can't, then it's stuck like that. Not sure why but resetting the pram and nvram doesn't change it. Only fix I found was to boot off cd, I don't know why it works or why I tried it but I get video back to the original monitor. Also sometimes I have to hit the power button twice even to make it boot off the cd even holding the cd key.

I hope this helps.

Good luck.