How to check my preamp for a DC leak

I'll add that the preamp should be on when you make the test, but with no signal playing.

It also might be worth checking your source players for DC leaks. Depending on the preamp design, there is the possibility the preamp is fine but is passing DC originating elsewhere.

Intermittent problems are among the hardest to run down. It could be an issue of a bad capacitor that only misbehaves when it charges to a certain point. Or it could be temperature related or tied to the type of signal. Cold solder joints are also suspect in this type of situation.

I'd measure for DC under all conditions - when it sounds OK and when it is misbehaving. There's a more than fair chance this is going to take some bench testing by a a good electronics tech.