I had this problem with my LT-40. At half throttle (.40 engine and 10-6 prop) the plane would climb like a skyrocket. It wouldn't pitch up, just gain altitude.

I reasoned that the wings were creating too much lift (imagine that?) and that this might be caused by having the ailerons drooped down a little, effectively creating more lift on the wing.

I took about 3 turns out of the aileron clevis's and this seemed to tame the problem. I still need to do more, but didn't want to get back into the wing again. I'll take 2-3 more turns out of the aileron before I fly it again.....

I was just thinking that this might be another reason for the rapid climbing that you're experiencing...

Bob