B767 Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP)
Flight Crew Type 833
Troubleshooting
Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Provided Assistance; Air Traffic Control Issued Advisory / Alert; Flight Crew Requested ATC Assistance / Clarification; Flight Crew FLC Overrode Automation; Flight Crew FLC complied w / Automation / Advisory; Flight Crew Regained Aircraft Control
Aircraft; Human Factors
Aircraft was deferred with Thrust Management inoperative. Approximately 40 miles east of ZZZZZ; [we] were given climb to FL340 from FL320. Used FLCH [and] added power manually to QRH climb power for altitude/TAT - observed ALT HOLD annunciation; ALT HOLD light on MCP and aircraft leveled off normally. As I was fine-tuning the N1 to achieve the correct Mach number; I saw the altitude warning light illuminate on the baro altimeter with aircraft descending thru FL33.8 with flight director above; autopilot light was engaged and I think the ALT HOLD light was too but not 100% sure. With assessing situation and reaction time; the plane had descended to 33;700 when I disconnected the autopilot and hand flew to FL340. Lowest altitude deviation was approximately 350. Airspeed was .805 (I know because I was fine-tuning it to try to get to target of .81). I went on break shortly after and Captain had call with Maintenance and they suspected MCP issues based on AP checks Maintenance asked the Captain and Relief Officer to do. (Aircraft was wandering in altitude with VSI swings of +/- 300-700 with different AP selected). ZZZZ did ask about our altitude deviation and we said we were having flight management issues.
[Report narrative contained no additional information.]
B767 pilots reported an altitude deviation due to a malfunctioning autopilot.