My 1st guess would be that you are experiencing the effect of Pump heat. When the pump runs it energy changes to heat and if there isn't sufficient heat loss the water temperature slowly keeps rising.  This usually is a problem when the weather is warm.

When the temperature is 1 degree above the set point, the Heat LED should be off and there should be no voltage across the heater element terminals. Measure the voltage and if indeed there is no voltage present, this confirmas it is a pump heat situation.

Dieter Jung

Northern Lights Cedar Tubs Inc.