When you say "keeps going downward" do you mean you have to add down trim in the elevator for level flight? If that's the case your stab or wing incidence is off. If it's a high wing plane shim the trailing edge of the wing up a tad to give it less positive incidence. I assume the stab is glued in and would be hard to adjust at this point. If it's a low wing plane you might have to modify the wing saddle a bit to raise the trailing edge. If that's not what you're referring to, please clarify your question with a bit more detail.