B737-300 flight crew reported autoplt trip off and altitude excursion occurs while en route.

Attributes
ACN	517890
Time
Date	200107
Day	Fri
Local Time Of Day	1201 To 1800
Place
Locale Reference	navaid : spa.vortac
State Reference	SC
Altitude	msl bound lower : 28600 msl bound upper : 29000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tower : dtw.tower
Operator	common carrier : air carrier
Make Model Name	B737-300
Flight Phase	cruise : level
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : multi engine pilot : instrument
Experience	flight time type : 159
ASRS Report	517890
Events
Anomaly	altitude deviation : excursion from assigned altitude non adherence : clearance
Resolutory Action	flight crew : overcame equipment problem flight crew : overrode automation flight crew : returned to assigned altitude
Supplementary
Problem Areas	Aircraft
Primary Problem	Aircraft

Narrative:

While in cruise flight at FL290, the autoplt dropped off line and the aircraft descended to FL286.

Original NASA ASRS Text

Title: B737-300 FLT CREW RPTED AUTOPLT TRIP OFF AND ALT EXCURSION OCCURS WHILE ENRTE.

Narrative: WHILE IN CRUISE FLT AT FL290, THE AUTOPLT DROPPED OFF LINE AND THE ACFT DSNDED TO FL286.

Data retrieved from NASA's ASRS site as of July 2007 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.