A319 first officer reported autopilot did not maintain altitude during cruise flight.

Attributes
ACN	1678079
Time
Date	201908
Local Time Of Day	0001-0600
Place
Locale Reference	ZAU.ARTCC
State Reference	IL
Environment
Flight Conditions	VMC
Aircraft 1
Make Model Name	A319
Operating Under FAR Part	Part 121
Flight Phase	Cruise
Flight Plan	IFR
Component
Aircraft Component	Autopilot
Person 1
Function	First Officer Pilot Not Flying
Qualification	Flight Crew Multiengine Flight Crew Air Transport Pilot (ATP) Flight Crew Instrument
Experience	Flight Crew Last 90 Days 225 Flight Crew Total 4692 Flight Crew Type 2797
Events
Anomaly	Aircraft Equipment Problem Less Severe Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Clearance

Narrative:

During cruise flight the autopilot commanded a descent like it was trying to capture a path from above. The aircraft descended below the selected FCU altitude so the captain turned off the autopilot and arrested the descent where we leveled off about 1;100 feet below FCU selected altitude of FL360. We informed ATC and climbed back up to FL360. We noticed the FMGC 2 was going blank and then coming back on so we used autopilot 1 the rest of the flight even though I was flying.

Original NASA ASRS Text

Title: A319 First Officer reported autopilot did not maintain altitude during cruise flight.

Narrative: During cruise flight the autopilot commanded a descent like it was trying to capture a path from above. The aircraft descended below the selected FCU altitude so the Captain turned off the autopilot and arrested the descent where we leveled off about 1;100 feet below FCU selected altitude of FL360. We informed ATC and climbed back up to FL360. We noticed the FMGC 2 was going blank and then coming back on so we used Autopilot 1 the rest of the flight even though I was flying.

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.