Aha! Similar problem here; Mini (Intel, Macmini2,1, the 2007 version with the larger HDI port not the smaller one)
Problem seems to happen only with both HDMI and USB connected (USB only to use the built-in webcam on the monitor, an AOC V22+ (which I love, it's a very nice monitor; LED instead of fluorescent backlight).
Sometimes it would suddenly switch to where the dock and menu bars and status bar were hidden (the desktop gets 'too big for the screen).
Then I'd go to the Display settings and unselect "overscan"
Then it'd be OK for a while, then switch to a big black space around the desktop window, lots of unused screen).
Then I'd go to the Display settings and reselect "overscan" and it'd be Ok.
For a while.
There really ought not be a need to fiddle with 'overscan' at all -- since I'm not using this as a television. Mac OS 10.3.6 seems to mistakenly think the monitor is being used as a TV -- maybe when there's a video in a browser window?
Not sure.
Anyhow it becomes a rock-paper-scissors problem where I have to keep chasing the settings to undo whatever it changes to at random, Lather rinse repeat.
Seems like it's only a problem for me when I have the USB cable also connected, to use the built-in webcam -- so I don't use that now.
The AOC folks have been working hard on solving this.
I just found this topic and have referred them here.
They'll be glad to know it's not their problem, I guess.
(does it happen on Apple brand displays too? I'm not sure)