I have had this problem. other than the engine you may need to reset the horizontal stab and main wing incidence.
If the elevator is nuetral your plane should fly level under 3/4 power and climb only when power is added, when the throttle is cut the plane should continue straight ahead then as it slows down start to lose altitude but not flair up or down.

for starters set your engine straight ahead............understand I say this not knowing what you are flying. a high winger will react a bit different than a low winger - several variables apply dependent upon the airplane type. with the set up you presently have you engine is fighting against the lift of the wing and should be shimmed with washers for less down thrust. but when you do the plane will tend to loop because you leading edge of the wing is too high or the horizon stab. leading edge is too low.

In a nut shell your bird is out of trim.