Since it's an intermittent fault (i.e. sometimes works), it could be a dry or cracked solder joint, maybe on something that can get subjected to mechanical stress, like a socket or header pin.

I have an A-106-6 filter, which I bought used.  When it arrived, one of the outputs didn't work at all - which happened to be the first one I tried.  I resoldered the pins for that socket, which cured it, and it's been fine since.

Dry solder joints are often visibly different to other joints on the board - they might look duller, or more uneven.  In my case, the problem one didn't have much solder on it at all, so someone at the factory had an off day. 

- Andy