Well, it's really not "under load" when on the ground. Sure, the prop turns, there is resistance, and some prop drag, but not like in the air. The engines load up differently, the props work differently, and it all ads up to different in the air as compared to on the ground. I don't have an answer for you because all that hs been mentioned already, is what I would do. Someone will come up with a good suggestion that will certainly help.

You also might think of having someone with experience at your club field look at it. They might be able to help you clear up the problem.

CGr.