I don't think its the dimmer. The dimmer is like 2 weeks old, but its possible that it could be defective. Still i think there may be a prob with the flexwatt.

I flipped over the tub and hit the flexwatt with a IR thermometer and got different temps all over it ranging from 96 down to 82... its not evenly heating.

I also moved him to a different area of the room because he was about 7 ft away from a window (but its usually closed). I moved him somewhere that there is no direct line between the tub and the window