That airplane has a lot of draggy stuff below the thrust line and the thrust line is above the wing C/L. Drag is not linear, rather a function of velocity squared.

Quick test - fly it without the landing gear. I bet the problem goes away or is muted dramatically. With the motor above the wing, its already trying to tip it on its nose, as is the landing gear's drag from below.

Larger fixed horizontal stab area might counteract these tendencies some.