Temp display bouncing around

It is the season. I have a PDC Spa that is about 10yrs. old. My problem is that the temp. on the display is bouncing around from about 100 to 104 causing the pump and heater to kick on and off. Been researching this morning and the 2 parts I have looked at are the heater unit and circuit board. I am assuming the thermostat is part of the heater (http://www.advancedspaparts.com/b25255020011.html). If it is the circuit board I don't think I will be fixing at over $400. Any input would be appreciated.