Iowa Steve

I have a 700 Series Vita, about 10 years old. Has been a great tub overall. About a month ago, (with tub set at 102 degrees), found the water temp reading at 99. The circ pump was running but the heater indicator on the control panel was not lit. If I press the "temp up" button, to 103 or 104, the heater indicator on the control panel appears for 2 seconds and goes off again. (and it is not an indicator problem as I can tell the tub is not heating) if I come back a few hours later, the tub will be up to temp again, and if I adjust the temp up or down, the heater indicator goes on or off accordingly. this is a daily occurrence now. I have reset the tub to factory specs by turning off power for 5 minutes and then holding clear for 10 seconds per the manual. I have replaced the temp sensor, no change in symptoms. so, 1/2 the time the tub functions as designed, and 1/2 the time, it will cycle down to 97 to 99 degrees and not kick on the heater, or when I manually adjust the temp higher. so, my issue is one of a few things (I think). Either there is a "time issue" that it will not function in some time window, or it is a limit issue...ie; the tub runs and heats for a period of time and then it shuts down until something cools off and resets?, or just an intermittent "fault" somewhere. scratching my head.... (and of course this is happening in the middle of an Iowa winter..hoping not to convert my tub into a 300 gallon block of ice)