Hi K Hunt,
Would believe your problem is basic. Probably in the design
force arrangement.
The lift of the wing and horizontal tail are not in proper proportion.
Surfaces lift at a square of the speed.
When speed changes so does the lift in that respect.
An aircraft should fly level at full power and stay level with reduced power until lack of sufficent power reduces speed to the
point where lift is no longer sufficent to overcome gravity.
At that point the craft decends.
When the tail-wing lift proportion is correct the craft will remain
level throughout a major amount of power change.
If your craft flys level at full power with elevator trim neutral and
baloons at low power the proportion is off in favor of the wing
In short the tail is not lifting sufficently.
The tail lift needs to be increased. Lift is generated by these factors> area, angle of attack to the line of flight and airfoil.
A simple try would be to add positive incidence to the horizontal
tail.
Note> when tail lift is increased the neutral point moves back.
Thus the balance point must be moved back to compensate.
This sort of thing seems to be a problem for others also.
I have already posted the factors in detail on another post in this
forum if you would be interested.
Good luck and let me know how you make out, OK?

Hal [email protected]