Old Raypak Versa 265b heater goes on and off when heating spa, but stays on when heating pool. Temperature sensor or Unitherm Govenor??

Hi, I have an Old Raypak Versa 265b pool/spa heater that I hadn't turned on in over a year. I started it up the other day and it was fine, but heats the spa for about 10 mins then starts turning on and off every 5 mins. If I heat the pool with it, its stays on just fine, which leads me to believe their is an issue with the temperature sensor? Being that the pool is 20,000 gallons, so unless I run it it for 48 hours straight the water is never getting above 70, as compared to the...

Old Raypak Versa 265b heater goes on and off when heating spa, but stays on when heating pool. Temperature sensor or Unitherm Govenor??