Heater/pump running when water already at set temperature

[Chappy]

I have a 6 year old Marquis 545, and have 90 minute filtration cycles set to run once in the afternoon and once after midnight. I've recently noticed the pump is running for extended periods outside of these cycles, and when I check find the the display indicating it is heating even though the temp display shows the desired set temperature. The spa is in some direct sun and during the hot summer there's no way the spa is losing heat during the days when the heater is regularly running, and in fact the external heat often leaves the water temp higher than I have it set. I'd like to ask for any knowledgeable advice as to whether this excessive heating is likely an issue with a temperature sensor/thermostat, or the control unit?  I live in a rural area and am mostly on my own for repairs.  Thanks in advance.