Charlie Yahr

I have a Vita Spa L700. Ive had a problem lately where it won't keep the daily temperature. I can set the heat for a day, and it will heat to that temp (100 degrees) and hold it to the end of the day. The next day I go out to use it, the temp on the display is flashing. I can adjust the temp setting which is still flashing 100, but the water temp is lower, typically it shows low 90's. The only way I can get it to heat up again is to run the system diagnostics by pressing the "Prog" button 3 times. It will then run the 3:30 minute Diagnostic. After that has finished, I can then set the temp to the daily temp again and it will heat up for the day. Next day I have to do it all over again. Anyone else having this problem or know of a solution?