Either tail heavy or too much down thrust.

Tail heavy if:
when you fly full throttle and then cut it it dosen't immediatly balloon, but balloons as the plane slows down. Try adding some nose weight to see what happens. Or just force yourself to push on landing. A nose heavy plane will do the same thing in reverse. Look up the term "flight path stability" if you have an aerodynamics book.

Too much down thrust:
if the change in elevator trim required is immediatly correlated with throttle possition.