It sounds slightly familiar. I had an early seagull Harrier 90 and put a Saito 90 in it. Just bizzare flight charactaristics. Wierdest was trying to land the thing. Pull the power back and it jumped 15 ft higher into the air! I finally gave up on it not wanting to kill such a pretty plane. I sold it to a returning competative pattern flier. He also found it challenging. Then after a flight he picked it up by the prop and the motor moved. Everything was tight. 20 minutes later it didn't move. He started it and ran a tank. Picked it up by the prop and the motor moved. The motor mount became soft with the heat of the motor! He switched to an aluminum mount and was eventually able to tame the beast. I have flown it since and the major problems are gone. Lots of mixes required to make it fly well and its still a long way from flying well. Not all that great for 3D, especially when compared to profiles. I did a Harrier 46 with two stroke power (st .60 then OS .46 AX) and no issue. Plane doesn't 3D with junk with either power plant. Just too many mixes to do to get a neutral flying plane to make it worth it. Its gonna go away too. I'll stick with the profiles and bigger electrics for 3D. Anyway, check the mounts under heat.

bob branch