What was found
(CAN) AT TOP OF CLIMB, FL210, 30 SECONDS AFTER REDUCING POWER TO CRUISE SETTING, BOTH ENGINES SUFFERED MULTIPLE EVENTS OF TORQUE DROP OFF FOLLOWED BY A SURGE BEFORE STABILIZING AT SET TORQUE. PILOT REPORTS THE SURGES WERE ACCOMPANIED BY VISIBLE FLAMES FROM THE EXHAUST STACKS AND REPORTS THAT THE ACFT WAS IN TURBULENT, ICING CONDITIONS. AUTO IGNITION WAS ON AND ICE VANES WERE DEPLOYED. AFTER 3 MINUTES OF MULTIPLE SURGES ON BOTH ENGINES, POWER WAS REDUCED AND A DESCENT INITIATED. THE ACFT LANDED UNEVENTFULLY.