MD80 FLT CREW HAVE SEVERAL ALTDEVS OCCUR WHEN THE #2 AUTOPLT MALFUNCTIONS AND CAUSES CLIMB AND DESCEND OSCILLATIONS.

Narrative

APPROX 2 HRS INTO THE CRUISE PHASE OF THIS FLT (CONTINUOUSLY SMOOTH CONDITIONS); THE #2 AUTOPLT BEGAN AN UNCOMMANDED 1000 FPM CLB. MY FO AND I BOTH NOTICED IT ALMOST IMMEDIATELY. (ADDITIONALLY; 'AUTOPLT TRIM' WAS ANNUNCIATED ON BOTH FMA'S DURING THIS EVENT.) MY FO (PF) REACTIVELY REACHED FOR THE VERT SPD WHEEL ON THE DFGS PANEL. HE QUICKLY AND VERY SMOOTHLY INPUT A VERT SPD DSCNT OF ABOUT -500 FPM. THE ACFT REVERSED ITS CLB INTO A DSCNT. THEN; SUDDENLY; THE VERT SPD CHANGED (WITHOUT INPUT) TO -2500 FPM. AT THIS TIME I STATED 'JUST DISCONNECT THE AUTOPLT' AND RETURN TO FL310. DURING THIS YAHOO; THE ALT WENT ALMOST TO FL312; THEN TO FL296-FL297 (I SAW FL296.5). ALMOST SIMULTANEOUSLY; CTR (ZFW; I BELIEVE) CALLED US ON THE ALTDEV; AND EXACTLY AS I SAW IT. WELL; MY FO HAND FLEW FOR 15-20 SECONDS AND RETRIMMED THE ACFT; THEN ENGAGED THE #2 AUTOPLT. THE AUTOPLT HELD PERFECTLY FOR APPROX 2-3 MINS. THEN; ONCE AGAIN; SUDDENLY BEGAN AN IMMEDIATE 1000 FPM VERT CLB. AT THIS TIME; MY FO DISCONNECTED THE #2 AUTOPLT; STABILIZED THE ACFT; AND THEN ENGAGED THE #1 AUTOPLT. THE #1 AUTOPLT WORKED FLAWLESSLY FOR THE REMAINDER OF THE FLT. I ASKED CTR IF THIS HAD CAUSED ANY CONFLICT AND HE STATED NO. UPON GATE ARR AT DTW; A MAINT ENTRY WAS MADE AND MAINT WAS CONSULTED. THE MECH REVIEWED THE FAULT HISTORY AND FOUND A HISTORY OF PREVIOUS FAULTS IN THE AREA OF DFGS ALTDEVS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT HE HAD PERFORMED SOME RESEARCH ON THE ACFT AFTER THE INCIDENT OCCURRED. HE STATED THAT WHEN HE AND THE FO HAD PREFLTED THE ACFT; THAT THEY WENT BACK 3 DAYS INTO THE LOGBOOK FOR ANY UNRESOLVED ACFT PROBS. NEITHER ONE OF THEM SAW ANYTHING RELATED TO AN AUTOPLT PROB. THEY SPOKE WITH A MECH LATER WHO STATED THAT HE HAD DONE A BITE CHK ON THE ACFT'S COMPUTER AND FOUND THAT WITHIN THE PAST 5 DAYS THERE HAD BEEN A HISTORY OF 5 ALTDEVS. A WK AFTER THE INCIDENT THE RPTR NOTED THAT THE ACFT WENT BACK TO THE MAIN MAINT BASE; PERHAPS ON A FERRY FLT. THE ONLY INFO HE COULD FIND RELATED TO THAT MAINT PROC WAS A REF TO AN MEL 08-1295CC AND A REMARK REGARDING THE DFGS BEING MENTIONED WITH AN 'OPTION ONE' REMARK AND 'UNCOMMANDED ROLL' FOR AUTOPLT #1. HE SUPPOSED THAT THE ACFT WAS RETURNED TO SVC AFTER THAT.