B737 Next Generation Undifferentiated
Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine; Flight Crew Instrument
Flight Crew Last 90 Days 195; Flight Crew Total 5265; Flight Crew Type 5265
Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance
Flight Crew Returned To Clearance
Aircraft
At FL350; for no reason; aircraft went into control wheel steering and started descent. Disengaged autopilot; corrected situation and notified ATC. No TCAS. Aircraft stable for duration of flight. Made logbook entry.
B737NG Captain reported that an uncommanded autopilot mode switch resulted in an altitude deviation.