This sometimes happens to me with vga on my dp533. never happened on my b&w g3 so I think it must be a hardware issue (not software fixable) with both adc and vga on the card.
Seems it just can't tell there is a monitor connected so it defaults to 640 x 480, which you'll usually get if you boot up w/o a monitor and then plug on in.
It isn't the greatest solution in the world but what I do when it happens (usually after a crash and restart) is to simply shut down, turn off the monitor, let it sit and then turn it back on a good 5 - 10 seconds before I boot up just to be sure the computer can find it.
Could work, maybe not, try it
