Probably you should prop the fuselage up on a level table to be what you would expect to be 'a flying condition', see if the front tip of the wing (front most part of the leading edge) and back of the trailing edge of the wing are 'pretty close' (within reason), check to see if the stab is horizontal and see if the prop is close to 90 degrees to the table. You might have an error in each of these areas so that each contributes to poor handling.

If the plane glides in for a landing with no problems (with the engine at idle or completely OFF and just a little up elevator) but with power it acts very odd then the engine thrust line probably needs adjusted.

Maybe you can update this thread when you have some observations and let us know what you finally changed.