If the fuel tank is high, and you are running excessively rich to compensate for leaning out when inverted... it might partially explain it... but 4-strokes don't usually change rpm much in flight. (assuming constant throttle setting of course...)

There is a possibility the fuel tank is moving and pinching the fuel lines... (again if you are running quite rich, the rpm change will be visible... set correctly, it would lean out and die.)

I'm running the newer version, O.S. .91 FS II, and even when I was breakng it in it never exhibited anything like you describe.

Get a second person to help... don't start the engine.
set the throttle at the position where you note the most change in flight.
Have the other person slowly tilt the plane as you watch the throttle linkage at the carb.
I'll bet you see the throttle moving. You might notice that the carb mounting screws are loose.... You may find the fuel tank shifting (have it about 3/4 full)
If you don't see some movement just tilting the plane, shake it gently as it is moved, to simulate the engine vibration in flight.

which prop are you using? What fuel?

Yes, the engine will get much better as it breaks in. I've found with the newer engine that a 14X4 (Zinger wood prop. Haven't got hold of the APC 14X4 W yet...) is getting max torque and max vertical climb. I'm running 10% 2-stroke sport fuel, which has some castor oil. (I was trying to hover and torque-roll a Fokker Dr1. Not quite enough power for that with the 9.5 lb plane.)