Retired eleco here.. Clothes irons were well known to trip GFi's. Nothing wrong with the iron after one of those "special technicians" tested them . Still tripping. I had a look and there was fine lint in the lead connection compartment. ( terminal box) . Blew it out with air and no more problem. Have a look in all of your terminal boxes.I have a similar problem. Motor and capacitors were tested by a motor shop, tested good. Problem was GFI pops when trying to start lathe, breaker does not pop. Broke the start switch trying to re-install it. Buy new switch, hire electrician. He worked for at least two hours , either the forward/ reverse switch is bad or the motor. Ordered new switch this afternoon.
No way in hades I'm going to move the lathe out of the shop. Just to remove the motor I had to raise the lathe with my shop crane. Bottom bolts on motor frame can not be accessed unless the lathe is at least 6" high so a wrench can turn.
While I'm waiting for the switch to arrive I decide to put the belts back on. The cogged timing belt is tough to install. Grizzly says in their manual to loosen the motor bolts, raise the motor and install timing belt. I did not want to raise the lathe again as it involved too much work. Backsplash, motor cover, and digital readouts have to be removed first. Instead I removed the upper pulley which is bolted to a steel plate I refer to as a banjo. The pulley shaft goes in a slotted hole, all I need to do is lengthen the slot so the pulley distance between the upper and lower cog wheels is a bit shorter . Probably will make the slot 1/2" longer. When I re-install the pulley will be set back to center of the slot giving me about 1/4" deflection.
Hopefully the switch is the problem or I will have to remove the motor again.