Typically a plane with an incident problem will change pitch with throttle changes. Tail heavy planes are very sensitive to elevator changes and will "porpoise" between nose down and nose up. I think you have an incident problem. Another test is gain altitude (probably not a problem at this point) and cut the power back to idle and see how she flies. If it is a tail heavy condition, the elevator will still be sensitive. If it flies normally with out the tail dropping out it is more than likely an incident problem. Try setting the top wing -1 degrees and make sure the stab is 0 degrees and the bottom wing is 0 degrees. I had a Christen Eagle years ago with the same problem, never did work it out.