That's great advice - thanks. So, I'd need to open up the power supply, to get at the power switch?

The monitor still twitches when the disk isn't plugged in though, so it's possibly something else. I've had a look at the ccadams site, and tried resoldering most of the analog board connector points, but alas, the screen's still twitching around.

If I lightly tap the side of the machine while it's on, this seems to resolve things for a while at least. So, am I right in assuming that this is probably a bad solder joint?