Some programs, particularly games, include a change of resolution in the program. The better ones will change it back when you close them; but I'm told that quite a few don't.

Something of this sort might be suspected if it happens only with one or two specific programs; but you implied it's pretty random.

If a particular program actually needs a specific resolution, it might "look right" so you wouldn't notice the change. If that program is "suspended" or just minimized, another program could look lousy and get the blame for changing resolution. If it is a "program forced" change, closing the program that did it might - or might not - switch things back to normal.

If you can't find something like this to blame it on, the next logical suspect would be a graphics card that's intermittently crapping out. This likely would mean the graphics card is close to dying.

Your "card" may be via "integrated components" on the mother board or it could be a separate card in a slot. A failure of "on the board" graphics components usually doesn't kill the rest of the mother board so you usually can just plug a replacement into a slot.

If it's a separate card, connecting the new monitor might have wiggled the card, and it may just need a few more wiggles to clean the contacts where it plugs into the card connector; but recent graphics cards (or graphics components on the MBoard) tend to run pretty hot, so they seem to age faster than other kinds of components. I've replaced about three (on two computers) in the past couple of years, with failures coming at about 6 to 10 years of 24/7 running with big CRT monitors.

If the card gets hot, and "fails hot," the computer should revert to the "no card" driver that it uses (usually) for Safe Boot, which is a low res display at least during startup. Most Windows OSs might restore the card when it cools off enough to start working again(?) since it will "retry" problem components/drivers in some cases.

There probably is an "error log" on your computer that might tell you something, or that you can turn on to log some clues, but I can't give very good generic help about where to look for any such goodies. System Information (Start|Programs|Accessories|System Tools|System Information) has one "Errors" log in the "Software Environement" section that might have a clue, although I'm not sure it's the best place to look.

John