Hmmmm again....

This is all very hard to diagnose over the internet let alone without seeing the model flying but I wonder if you aren't just getting the rudder so far over that the drag of the slideslip (yaw) and the deflected rudder aren't just too much for the engine to handle and the model is acting like you put on a speed brake.

To help answer this how well does it do a wingover. You know, vertical climb, throttle back, cut in full rudder, add blip of throttle to blow air over rudder to kick model over to one side. If a stall turn like this is pretty much a pivot and down the same up path then the rudder may just be yawing the model so hard that the drag is too much and the model settles due to lack of speed.

What size engine is on there and what's the wing area and weight?

Oh, and I know this is a personal taste issue but I go along with Ben about the need for a cowl. Or at least the lower part of the nose could have been angled up more to help the aerodynamics.

Is this an original? If so it's pretty darn good other than these few niggling details.