I have not seen this exact problem.

I have two DFC-230's which have exhibited various weird problems. In
both cases, the solution was to add star washers between the circuit
board and the mounting tabs. Over time, the "grounds" provided by the
contact between the solder and the mounting tabs oxidizes and becomes
high resistance. The star washers make a gas tight connection.
Once this was done, the problems went away and both units have
performed flawlessly.

Tim AA6DQ