Display taking too long after monitor turned on

It takes about 20 minutes to get a picture after turning my monitor
back on in the morning if I've left my computer on all night.

But if I decide not to wait and reboot instead, the picture comes back
immediately. (Same thing on those times when I did turn the PC off
the night before -- when I cold boot the PC and monitor together, the
display comes up right away.)

Does anyone have an idea of what's happening here and how I can fix
it? This is a pretty recent development, and I can't seem to pinpoint
the cause. My graphics card is an ATI All-in-Wonder 9200. My OS is
Windows XP with SP2, with screen saver and power management both
disabled. The only advice ATI has for me is to update the drivers,
which had no effect.