I definitly believe you have a problem with the decalge,(difference in angle of incidence between wing and stab). Not sure of the plane you're referring to. I do know of one named Contender a low wing fully symetrical wing designed perhaps 30 years ago if that was the one. If however it is another with a flat bottom airfoil they will definitly cause problems with major speed changes since designed to be flown at low speeds.Try to set wing, stab at zero and engine 2 degrees down thrust. when flight testing check what it does at both upright and inverted flight. Should maintain level with none or very little trim adjustments and at all speeds other than very low.