As a general rule of older electronics - capacitors have the shortest life expectancy, especially the electrolytic ones (the cylindrical ones). If you see one with a bulging top, it should be replaced. Some even visibly leak their innards on the pcb.
I also had an O2 with a similar problem. Worked fine for a bit, but got funky after it warmed up. I never got around to fixing it, though. I think I used it for donor parts, actually.