There are at least three possibilities (although I'm sure you've eliminated a number of them):
(1) Your heater has gone the way of the dodo (your hypothesis)
(2) Your thermometer is wacky
(3) One of your pumps or other embedded electrical equipment is going the way of the dodo
Yep, I agree with Hy on this. What brand, and also important is what wattage? You may do well to divide heating duties between two or more heaters, rather than have just one (especially if it works out to 4+ watts/gallon).
Ok... I looked at all the things that could be wrong... I need a new heater... It is completely screwed up... The thermostat does not seem to work... I got a new one and the temp seem to be all good now!