I have a Sta-Rite spa heater that shuts off before reaching desired temperature. I set the digial display to 100F but it goes off at 76.  After about a minute, ignitor might kick back on and get the temperature to 78 but then it shuts off again.

So far, I replaced high-limit switch, flue sensor, gas shutoff switch and thermistor.  Still no luck. I bought gas shutoff switch from Amazon Warehouse and it was 'open box'. I am getting a new one just to make sure that the one sent by Amazon was not in fact defective. If that doesn't work, I am out of ideas.  What else could be tripping the the unit?

Any ideas are welcome.

Thanks in adavance!