A320 CREW HAD ACFT UNCOMMANDED PITCH DOWN IN ZOB CLASS A AIRSPACE.

Narrative

AT FL330 ON AUTOPLT #1 NEAR MKG VOR; ACFT STARTED UNCOMMANDED FAST DSCNT. ALT ALERTER ON FCU SET AT FL330. NO REASON FOR ACFT TO START DSCNT. AUTOPLT WAS DISCONNECTED. ACFT DID NOT GO BELOW FL326. ACFT WAS LEVEL AT FL330 WITHIN 20 SECONDS. NO COMMENT FROM ATC. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE CREW ENTERED A DISCREPANCY IN THE ACFT LOGBOOK. ACR MAINT DID A COMPLETE SELF CHK OF THE SYS. ALL TESTED ITEMS PASSED THE SELF CHK; AND THE ACFT WAS RETURNED TO SVC. UPON LEARNING THIS; THE CAPT CALLED THE ACR CHIEF PLT TO VOICE CONCERN THAT NOTHING HAD BEEN DONE TO FIX THE PROB. THE ACFT WAS RETURNED TO THE ACR MAINT BASE FOR ENGINEERING EVALUATION. COMPONENTS WERE REMOVED FROM THE ACFT AND TESTED ON THE ACR MAIN COMPUTER SHOP TEST RIG. ALL TESTED NORMAL EXCEPT THE 'SEC COMPUTER' SPOILER AND ELEVATOR COMPUTER; WHICH SHOWED A FAULT. THIS COMPUTER WAS REPLACED AND THE ACFT HAS FLOWN WITHOUT FURTHER PROB. THE RPTR IS CONCERNED THAT MAINT RELEASED THE ACFT FOR SVC ON THE BASIS OF THE SELF TEST; WHICH SHOWED THE 'SEC' TO BE FAULT FREE.