B737NG captain reported that an uncommanded autopilot mode switch resulted in an altitude deviation.

Attributes
ACN	1718955
Time
Date	202001
Place
Locale Reference	ZZZ.ARTCC
State Reference	US
Environment
Flight Conditions	VMC
Aircraft 1
Make Model Name	B737 Next Generation Undifferentiated
Operating Under FAR Part	Part 121
Flight Phase	Cruise
Flight Plan	IFR
Component
Aircraft Component	Autopilot
Person 1
Function	Captain
Qualification	Flight Crew Air Transport Pilot (ATP) Flight Crew Multiengine Flight Crew Instrument
Experience	Flight Crew Last 90 Days 195 Flight Crew Total 5265 Flight Crew Type 5265
Events
Anomaly	Aircraft Equipment Problem Less Severe Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Clearance

Narrative:

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.

Original NASA ASRS Text

Title: B737NG Captain reported that an uncommanded autopilot mode switch resulted in an altitude deviation.

Narrative: 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.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.