Do you have any down thrust in the motor? I am guessing you don't. A high lift trainer wing will really want to go up with no down thrust in the motor.. if the motor has a little upthrust it will REALLY climb.. Also next time you fly it take some cardboard and shim the trailing edge of the wing up a bit, basically add some down/negative incidence in the wing..That also might help...
When you trim it out for high speed flight does the plane glide or DIVE when you pull back to idle? If it dives I think thats another sign you have up thrust in the motor..