When you tested J107 to the flash lamps and it metered Ok, did you then test the flash lamps and they were working?

I doubt U3 would be intermittantly faulty (it is possible, but low probability).  More than likley a loose connection or bad solder join would cause the fault you describe.

If it was all the flashers excluding the Plane Gun that didnt work then this points to the 20v being missing.  So most likley a connection somhere or a bad solder joint or connector on the power driver/supply board that provides the 20 volts to J107.