I have a Rheem 266A and it has been functioning great for about four years. Recently, the temperature display on startup quickly - within minutes - climbs to the set temperature. 78 to 90 in less than two minutes. Unfortunately, the pool temperature is no where near what the reading says, and then the heater shuts off. The heater then will then start up again and go through the same cycle continuously. Water outputting to the pool is hot and there is no error on the display. The PSI from the multi-valve says 20. That has been the normal reading, however it feels like the water pressure outputting into the pool is less, but enough to keep the heater on. I have backwashed the filter several times. So, I heard that it is either a faulty sensor, dirty sensor or still a dirty sand filter. Please help... thanks!