Apple's late-model CRT-based systems seem to have numerous problems like this. I've got an original iMac with a failing flyback, and one of the original B&W G3 companion displays with bad solder joints. So, your problem is not uncommon, and it's likely that re-soldering things will get you back to normal. However, if you've never worked on the actual CRT section of a compact Mac or anything before, I can't recommend doing it yourself until you're trained properly.
Also, the overclocking thing is tricky, but not likely to cause your issue. One thing to note about the 83MHz bus is that controllers capable of running at that speed were actually installed on Rev. A G3 boards, though none were ever clocked as such. If you can verify that you've got the 83MHz part in there, then you can pretty much rule out OCing as your culprit (unless you've got the processor upclocked too much).
If you're opening it up anyways, make sure to check all of the cables, particularly those on the A/V riser card, as they may have worked themselves loose.
Also, the overclocking thing is tricky, but not likely to cause your issue. One thing to note about the 83MHz bus is that controllers capable of running at that speed were actually installed on Rev. A G3 boards, though none were ever clocked as such. If you can verify that you've got the 83MHz part in there, then you can pretty much rule out OCing as your culprit (unless you've got the processor upclocked too much).
If you're opening it up anyways, make sure to check all of the cables, particularly those on the A/V riser card, as they may have worked themselves loose.