ORIGINAL: Schnap-Roll
It sounds like to much down thrust in the engine to me. Full power and too much down thrust will cause the plane to pitch downward, remove the power and it will pitch up.
I'm voting for down thrust too. If you trim the elevator for level flight under full power.... at half power you have half the power pulling the nose down.... and with all that elevator up-trim.... well, you get the idea.
I don't think the CG has anything to do with it unless it's grossly off..... And if it's set to spec like he said, it should be close enough. A little CG "tuning" isn't gonna fix this one.