This is usually due to filter cycles being too long, the pump operating for long periods of time will start transferring heat from the pump into the water heating the pool. This is more likely to happen in summer with warmer weather.
Filtering of 4-8 hours a day is sufficient for most pools.
Balboa Models
Changing Filter cycles Balboa TP600 - see here
SpaNET Models
Changing SpaNet filter cycles - see here
You could also try an EPRM reset - see here
SpaQuip / Davey Models
Changing Filter Cycles on the Davey SP601, please see page 6 of the manual below.
