The trimmer potentiometers on the 310 are not very robust. They are not designed to be turned many, many times. If a pot is in an audio circuit, it’s easy to hear when it is going bad. When it is in a control circuit, like the FET BIAS pot in a DAP, it can be difficult to diagnose as a source of a problem.
Recently, a 310 that I refurbished would periodically slam the low frequency meter downscale. The problem was very intermittent. As I was going through the final alignment, the FET BIAS control was very touchy. At that point, I decided to replace the pot. When I did, both problems disappeared. It was immediately obvious that the pot had failed.
Most of the pots in the 310 are Bourns 3386H-1-202 or equivalent. If you’re replacing caps and bringing a DAP back to life for long-term use, you might want to go ahead and replace the pots. They’re very easy to replace thanks to the single-sided printed circuit boards.