Your statements here need some analysis. Your first statement in the Original Post stated that the airplane just doesn't like to fly level. Several things can cause such a situation.
1.) A CG aft of 30 percent of the mean aerodynamic chord (MAC) will start creating such. For a TF Contender, the wing is straight enough to just use the average chord of the wing (chord at midpoint of the wing panels) to obtain good information. Since I don't remember that exact distance, let's say it is 10". If the BALANCE POINT (CG) is aft of 3" back from the leading edge, you are moving back into trouble city. Any point between 2.3" and 3" will not contribute to the situation you have. Aft of 3.3" will be moving into serious instability.
2.) Incidence differential: Regardless of all the hype reference some imaginary horizontal axis line of a fuselage, in convergent airflow (sub-sonic) the true datum line that sets all incidences is the Stabilizer Chord line. It is Zero. Aerodynamically all true incidences are then relative to that specific Zero.
3.) Flight Control Sensitivity: relates to the amount of elevator movement relative to your stick movement. Fast controls, especially if a short elev. horn in relation to a long servo horn, which moves the elevator very quickly and a large amount with slight deflection of the control stick.
4.) A very sharp wing leading edge or one that varies considerably in leading edge manufacture.

With all that, the contender is, IIRC, a symmetrical wing. That means if the wing and stab line are together the incidence is zero. Given the engine is zero with these two, the the aircraft will always be required to fly with some very small amount of up (or down if inverted) elevator. Back before all the mixes, etc., pattern fliers usually used about 1 degree of down thrust, which pulled the model down, then they trimmed a tad of up-elevator to compensate. When they rolled inverted, the elevator was seeking down but the thrust was pulling up, so the differences in trim were small enough to be resolved with very little if any other trim.
Now here you have placed 3 washers under your engine front. That provides lots of down-thrust at full throttle. You are trimming the elevator to compensate with the engine's nose-down force (loading the wing to down with less lift). Of course when the nose-down thrust is removed, the up-elevator takes command and UP SHE GOES. Welcome to aerodynamics.
My suggestion to you is this:
1. Be certain that your model balances laterally at a point within the 25-28% of the MAC. Distribute/add/remove weight nose and tail as needed. Remember that in most airplanes it requires about 4 Oz. of weight in the nose to compensate for 1 Oz. in the tail area. (depends on tail/nose moment arms)
2. Pencil in a line along the fuselage (forget incidence meters - have been flying very well CL and RC since before 1950 without them) that parallels the bottom of the stabilizer. Check that the centerline (chord) of the wing parallels that line. That is now zero wing/tail incidence. Set the engine to same parallel or to a maximum of 1 degree negative.
3. Check for warps with resulting incidence deviations. If not able to change, adjust your wing incidence in effort to zero out the deviations, or use trim tabs to compensate. That will take a few flights. (I have one 50" bi-plane, ST. .51, that I made some serious building errors. Wing turned out with significant warps. Using some trim-tabs, fixed and adjustable, that model is stable as a rock through all speeds, T.O. cruise. and landing. It can be done) Now that is all done, I think your machine should do well.
BTW: Check that your ailerons are parallell to the wing chord, or mabe just a SLIGHT down. If UP you need to adjust. It seems that one contender version had flaps. If so either SLIGHT droop but absolutely NO UP.

Now know that any aircraft trimmed for a given speed, will climb when more power is added, and start to descend when power is reduced. Lift being a function of the square of the airspeed increases or decreases with added power providing more speed or less speed when power is reduced. The given trim will TRY to maintain the given airspeed.

Contender Traits: The Contender horizontal stab - vertical stab. has a relationship that can cause some strange items. When straight and level, apply rudder either direction, watch it yaw in the applied direction, then roll the other way. [X(] This cause many pilots to constantly hunt for a better aileron trim which is not the ailerons, but caused by using rudder in maneuvering flight. A number of models do this and it is one of the first things I look for when into the first few flights of a new model.