Beechcraft pilot reported an altitude excursion due to an autopilot malfunction.

Attributes
ACN	1562764
Time
Date	201807
Local Time Of Day	0601-1200
Place
Locale Reference	ZZZ.TRACON
State Reference	US
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	Bonanza 36
Operating Under FAR Part	Part 91
Flight Phase	Cruise
Route In Use	Direct
Flight Plan	IFR
Component
Aircraft Component	Autopilot
Person 1
Function	Pilot Flying Single Pilot
Qualification	Flight Crew Instrument Flight Crew Multiengine Flight Crew Commercial
Experience	Flight Crew Last 90 Days 12 Flight Crew Total 2200 Flight Crew Type 221
Events
Anomaly	Aircraft Equipment Problem Less Severe Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Clearance

Narrative:

I was in level flight at 5000 feet MSL on an IFR flight plan talking to departure control. The s-tec autopilot was in altitude hold mode and had been in such for at least a few minutes. Suddenly; the autopilot pitched up the plane. I caught the altitude increase at approximately 5350 feet MSL. There was not a conflict or even any ATC communication. This is the second time; however; this has happened with this autopilot; but it was at least 20 hours ago. I had flown it and others had during this time with no problems. Approximately 20 hours ago; I was in the plane and the same thing happened in cruise. It was [at] a slower altitude increase and I caught it at about 125 feet altitude increase. I am reporting this to see if there have been other problems with s-tec ap's in altitude hold mode.

Original NASA ASRS Text

Title: Beechcraft pilot reported an altitude excursion due to an autopilot malfunction.

Narrative: I was in level flight at 5000 feet MSL on an IFR flight plan talking to Departure Control. The S-TEC autopilot was in altitude hold mode and had been in such for at least a few minutes. Suddenly; the autopilot pitched up the plane. I caught the altitude increase at approximately 5350 feet MSL. There was not a conflict or even any ATC communication. This is the second time; however; this has happened with this autopilot; but it was at least 20 hours ago. I had flown it and others had during this time with no problems. Approximately 20 hours ago; I was in the plane and the same thing happened in cruise. It was [at] a slower altitude increase and I caught it at about 125 feet altitude increase. I am reporting this to see if there have been other problems with S-TEC AP's in altitude hold mode.

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.