I would spend some time looking at the PCB with a magnifer and touch up any bad solder joints.
A common problem is fractured solder joints on a VFD. The pins are Kovar with some weird plating that does not solder great at the factory. At the far ends of the tube, the filament connection pins are high current and tend to be a problem at the pc board pads.
You can also take a plastic pen or poker and gently tap around to find the bad connection.