Is it giving problems when cowled or uncowled. If cowled, it may not be getting enough cooling. Air must go THROUGH the cooling fins. Having a big air inlet and a big air outlet don't do any good if the air can bypass the engine. You may need to baffle the engine to insure it gets good airflow through the fins.

Also, try it mounted upright on a test fixture. If it runs well, then it's the installation and not the engine.