I recently acquired a Cal Spa built in the mid 90's . I do not know the model number. I spent the summer rebuilding the frame and have been enjoying using it as the snow has finally come to Michigan. In the times I have run it I have had a hard time getting the temperature set. I would turn it up and the water would go up to 106 and then turn down until the heater would turn off at it would drop to 98. After a few days of adjusting though I would get it to 101 and leave it alone. That would work for months. A couple weeks ago I drained and refilled the tub. Balanced the chemicals and been again playing with the temperature again. Two days ago I got it back to 101 and it ran fine that way for about a day. Last night I came home and checked it as usual and it had soared above 125 degrees. Wow what an adventure that was getting it back down. The temperature had shut pumps down etc. This morning the temperature was holding at 100 and seemed fine again. I am thinking the thermostat control is going bad but wanted to verify with others who might know more then I. The topside controls are air buttons and after last night the air button for the light stopped working and the light is stuck on. The temperature control is a dial type mounted in the top side panel. The heater I believe is a 220v heater (Though it may be 110, the hot tub is 220 and all the pumps are 220, but I am not positive on the heater). The heater is mounted under the big black control box and abour 2" diameter tube. Thanks in advance for any thoughts you might have.

Definitely replace the thermostat.

http://spapartsnet.com/Electric-Heaters ... 8_0_1.html

Your heater is 220.