Originally Posted by Dionysius

Electrolytic capacitors will fail with age - they are prone to a leakage failure mode effect. When people claim they 'fixed' the board by soldering every land what they have really done is just touched the electrolytic lands with a hot iron and rejuvenated them with heat shock but this is a temporary fix IMHO. These caps were manufactured circa 30 years ago in a process inferior to what we would have today. You may find that not all have failed but my posit is the heavy hitter one which serves as the 'memory' in this ancient analog design is the one that is giving the major symptom. This circuit can be restored to its original and very fine original functionality. I admire you for looking at this and I will help where I can. The other caps act as filters (I am having to make assumptions here since I have no schematic) and their failure or partial failure could cause hunting and weird secondary effects. Note that these caps are polarity sensitive. And yes, you should replace all of the electrolytics. The other components are probably ok.