I turned the thermostat all the way up Friday night while in the Tub that had been holding 102 degrees regularly. Saturday Morning the tub was at 92. Heater is Watkins "no fault" 1500. This tub is a 1995 Classic 110 volt. Filters are clean. There is flow from the return but not very strong. Circ pump replaced about three years ago. Tried tripping GFCI and waiting 60 seconds for reset of Flow or high limit. Circ pump is vibrating. Any other ideas?

Try removing the filter.

Do you hear a "click" when you turn the T-stat all the way down and all the way up anywhere?

Also, service history: Have you ever had the heater replaced?

if it still has the cartidge heater it still has a flow switch. As swine mentioned, removing the filters or CHEMICALLY cleaning them could make a difference. Circ pump vibrating could mean an impellor off balance. It may have scale on it causing the vibrating. People have cleaned the impellor with some success. I beleive there's a manual summer time switch, looks like a toggle switch on the side of the black control box. Should still have tape on it in the OFF position. The switch should be in the off position. May need an inexpensive thermistor.