This event was a turbine stall .
 Continental says that the TSI 0550 B. engine should not be operated below 2500 rpm in cruise.  The reason is that cabin pressurization and other loads on the compressor/translate into too much load on the turbine blades causing them to stall.
A correction for this was made on the E engine.  I believe they said with this turbo we could operate as low as 2400 rpm.

Charlie K.