Scott,
Sorry, no clue here. Your warranty should cover any issue. Did you call your dealer/
You kink of freaked me out, since I just bought a J-355 about 6 weeks ago. It's been real cold here too... currently 28 F.
I just went out and lifted my lid and it's holding steady at 102. Once again, sorry... I have nothing to add except, that your Jacuzzi warranty is one of the best in the industry.... you paid for it... put it to work for you.
...edit... oops! Just noticed that you mentioned this was a used tub.