As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AT 40 NM FROM DEPARTURE AT CRUISE ALTITUDE OBSERVED A GRADUAL DROP IN LEFT ENGINE FUEL FLOW, % NG, TORQUE AND ITT. DROP STABILIZED AT ABOUT 80% NG AND 240 LBS/HR FUEL FLOW. DECISION WAS MADE TO RETURN TO SBA. AT 20 NM OUT FROM SBA LEFT ENGINE BEGAN TO SURGE IN POWER. DECISION WAS MADE TO SECURE LEFT ENGINE.