The most likely culprit on hot tubs and spas, is chemical abuse and the lack of ph monitoring that eventually destroys anything and everything that the water touches.
It could be your circulation pump, heater element, pressure switch, circuit board, temp/hi limit sensors... almost anything. Trying to do this third party is difficult enough, and with you in the middle it may be even more difficult. Don't know what to tell you, especially in your situation not being on site it could be a very trying task to evaluate something you can't see.
That's why I always ask for photos.
The first and most important step in any diagnosis is a visual inspection. Everything under the hood has to be evaluated first. I've visited many an installation where the customer told me over the phone they didn't see anything out of the ordinary behind the panel, and after arrival I discover that the circulation pump has so much corrosion and chemical crap all over it it's a wonder it's still turning.
So, take a step back and start with a visual, or someone else on the end of the line do a visual, look for stuff that didn't come with the spa when it was installed, post back and we'll go from there. Chasing a 'typical' solution to a common problem with any spa can be a costly answer to something that shouldn't have been missed with a visual.
That said, most of the time, model and serial numbers mean nothing to us, as these things change with the wind. We need to know:
1. Is the topside control an air type pushbutton, or an electronic type push button?
2. Does this spa have a small circulation pump that is smaller than the main jet pump, or just one or two large pumps?
3. Is the heater cannister one that has small hoses, or does it have a large 2" diameter tube attached to the back of the controller box?
4. A complete description of everything out of the ordinary that is observed on initial visual inspection... corrosive looking things are very important here.
Last thing, it's always better if you register on the board as you will have the ability to receive notification when we answer your posts.
Well, it's a place to start eh?
It's not much, but it's a start anyway.