If you have not done so already, place the wing on the plane as if you were ready to fly it. Then place it on a surface like a table, but make sure the table top is level. Set the elevator trim so that the elevator surface is level to the horizontal stab. Then shim the back end of the fuselage up until the horizontal stab is level.

Now check to see if there is any upthrust or downthrust. You might be able to 'eyeball' that by placing a piece of cardboard on the surface of the table that has a 90 degree edge (straight up edge). Align that straight up edge to the propeller when the prop is straight up and down. From a few feet away eye along the cardboard and see if there is any significant difference between the two vertical cardboard edge and the vertical propeller. If there is, try to determine the gap distance. This should be pretty close to no difference, but you might see a little bit of down thrust which is probably acceptable or even designed into the model.

Then measure the distance of the extreme front of the leading edge of the wing to the table and measure the distance from the trailing edge of the wing to the table when the ailerons are aligned to the airfoil correctly. How much of a difference is there? You might find a little higher leading edge measurement compared to trailing edge measurment and this might be acceptable and part of the design.

If all or these alignments look reasonable or are per the design then I guess it is still pointing to a balance problem or the plane is just not being flown fast enough.

If the stab is horizontal, the prop is perfectly vertical and the leading edge of the wing and the trailing edge of the wing is the same distance off the table then you are in the ballpark for being set right in those components. The wing could have a little decalage (the leading edge could be higher than the trailing edge) by design. And there might be a little down thrust by design. So you have to allow for these. But any gross differences should be suspect. Also a little error in each of these might add up to a problem in total.

I am assuming by 'it pitches down' you mean it continually tries to nose down more and more as you fly forward. I assume you do not mean the fuselage simply LOOKS like it is going down hill when the plane flies level.

If you set set the throttle at 3/4 of full then trim the elevator for level flight, then throttle up to full throttle, does the plane climb UP or does it being to lose altitude?