Hi stunt-man,
It could be caused by a change flow in the cowling, which the carb sees as a change in air pressure. This can cause an incorrect mixture and stall the engine. BME and others have special carb plates to correct this problem. Try flying the manuver without the cowl and see if the problems clears. If it does you can buy a new plate or make a shield of aluminum or solder a pice of brass tubing to the port. Mount the shield so that it blocks the prop blast without obstructing the port.

Dave