PM 3520b VFD acting up

I have no experience with your lathe, but lots with similar industrial controls. The results of your 'sharp tap' make me agree with PM tech support - - it certainly sounds like a loose connection. But it may not be a wire.

You indicated you checked the wiring. Did you check just the field connections to VFD (power supply and motor wiring)? Or, have you checked the I/O wiring as well (wires to start/stop/speed control switches)? A loose connection to these could also cause similar issues.

If you've checked all the wires entering and leaving the VFD, it's then time to look at any onboard wiring. Typical VFD has several sub-sections that may either be wired one to the other, or plug-mounted on a backplane (motherboard). Check for any loose plugs or boards that me be improperly seated. (Be safe, disconnect from power first.)

Last visually inspect for any damage or arcing on the component boards themselves. A surface mount component's solder or one of the foil surface conductors on a circuit board may be cracked. Your only sign of a problem may be some discoloration from any arcing that may occur across the crack. (Your tap could be just enough to close the crack, start current flow, cause the component to heat, expand, and keep the crack closed while in operation.)

If it is a board level failure, it may be an Easter egg hunt to find it, and simplest fix may to replace the VFD in whole or part. PM tech support can hopefully be more assistance if it comes to that, as they may be able to help isolate the issue to one component or board - - and not have to replace the entire VFD.

Edit - If a crack exists in the low voltage (5Vdc?) control circuitry, it may literally be microscopic in size, and impossible to see. Or, it could be in the solder joint under a surface mount component, and also impossible to see. Trouble-shooting is often a matter of remove-and-replace until the problem goes away. It is aggravating to replace a $100 board, due to a solder joint in a $0.002 resistor, but in today's world it cost more to trouble-shoot down to the component level than it does to simply replace the board (or entire VFD).

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PM 3520b VFD acting up