SOunds to me like you are stalling it.

The bigger the plane the less you can bang around the elevator without stalling it. If you this your first bigger plane then it will just take some time to get used to. You can't slam around a big plane like you can with little 40 sized planes.

This is also directly related to your wingloading. So if you have tried to stuff a bigger heavier engine in there you will stall easier.