What was found
CABIN ALTITUDE RED CAS MESSAGE ACTIVATED DURING CRUISE FLIGHT. CREW OBSERVED TEMP CONTROLLER WAS MALFUNCTIONING AND CABIN ALTITUDE WAS RISING QUICKLY. AN ODD SURGE IN CABIN PRESSURIZATION WHEN THRUST APPLIED WAS NOTED DURING TAKEOFF. ALL SYSTEMS APPEARED TO OPERATE NORMALLY PRIOR TO MALFUNCTION. CREW COMPLIED WITH REQUIRED EMERGENCY ACTION ITEMS AND DIVERTED. LANDING WAS UNEVENTFUL. FOUND THE CABIN AIR SUPPLY DUCT IN THE TAIL CONE TO BE DEFECTIVE. THE DUCT ASSY WAS R & R, THE PASSENGER OXYGEN MASKS WERE STOWED AND A SUCCESSFUL SYS, OPS CHECK WAS PERFORMED. ACFT WAS RETURNED TO SERVICE.