The heater element is the most common part to fail on a standalone hot tub. Looks like it is working for now. Usually it fails because of a small crack in the outer layer of the coil that can cause a short. I've had a few brands and they do not all show the same symptoms when the element starts going but most will disable the spa. They are end user replaceable if you are familiar with electricity and are careful. If you do not know which you have, you would need to kill power, disassemble the heater (two large plastic gaskets on each end of a metal tube, unscrew the two leads from the element and slide it out of the tube. There will be a PN on it somewhere and you can get them on Amazon or Ebay pretty cheaply.

Thanks...I installed a new 200 amp breaker box in the house as well as a 100 amp sub-panel in the greenhouse I built adjacent to the hot tub a dozen years ago.

It is working fine now...the temp is back up to 99. But I find intermittent problems to be the most vexing. If something breaks or quits working outright, I can usually figure it out. I can usually rebuild just about anything - and without any leftover and obviously unnecessary parts.... ;-)

Cheers