Okay, a year later, a few parts later, still a no go.
Here's the deal again; The spa is hooked up via 115. It will heat to 100 degrees exactly every single time I unplug it and plug it back in then stop and the green light will flash. If I plug it in at 75, heats to 100. If I plug it in at 92, heats to 100. Ran jet clean. The thermistors have been replaced...twice. The circulation pump has been replaced. It has a new filter. The circuit board and heater relay board have been replaced, unrelated, but new anyway. Even had a spa tech come out and he said straight up, I'll be honest with you, I don't know. I could throw parts at it but I don't really want to because I don't know what's wrong with it. So he was no help. Basically the only thing that isn't new on this spa is the control panel and the heater. But from what I have read, if it was the control panel, it would just flash right when I plugged it in, not heat to 100 first, but I don't know. I've replaced everything myself, which is significantly cheaper (like 80% cheaper) than having a tech do it after parts markup and labor. I'm not out all that much all things considered, so I don't mind replacing the heater, $300 on backyard plus or the control panel, but I dont really want to do both just guessing. Does anyone have any idea which one could be the culprit, or whatever other voodoo could be going on with it? I really don't get it at this point.