Okay, I'll be the first to hazard a GUESS. Nose pitches up when power is reduced...Maybe the motor is mounted with to much down thrust. Try shimming the motor so it does not point down AS MUCH. This may also be symptimatic of the extra drag caused by the floats. The faster you go the more drag they will produce... Again these are just GUESSES. Good luck.

PS: What is the wing span of that Cub?