B727 CREW HAD ALT AND HDG EXCURSIONS WHILE DEALING WITH AN ERRATIC AND MALFUNCTIONING AUTOPLT.

Narrative

APCHING MGM VOR; ARTCC ASKED OUR ALT. THE CTR ALTIMETER READ FL330.5. THE CAPT AND FO ALTIMETER READ FL332. THE IVSI APPEARED TO READ 100 FPM CLB. THE CAPT TURNED OFF THE ALT HOLD FUNCTION OF THE AUTOPLT TO DSND TO FL330. THE JET IMMEDIATELY AND AGGRESSIVELY PITCHED UP. THE CAPT QUICKLY DISCONNECTED THE AUTOPLT AND HAND FLEW THE JET BACK DOWN TO FL330. THE JET'S PITCH UP RESULTED IN A CLB TO FL334 BEFORE THE CAPT REVERSED THE AUTOPLT PITCH INPUT. MGM VOR WAS A TURN POINT ON OUR FLT PLAN. THE NEXT POINT WAS OTK WHEN THE AUTOPLT WAS DISCONNECTED AND SUBSEQUENT RECOVERY COMPLETED. THE TURN WAS MADE TO OTK. APPROX 1/2 WAY THROUGH THE TURN; ARTCC INQUIRED ABOUT OUR HDG. WE ENDED UP ABOUT 7 MI OFF COURSE. WE ASKED ARTCC IF OUR SIT MAY HAVE RESULTED IN ANY POTENTIAL CONFLICTS AND ARTCC SAID NO. ONCE BACK ON COURSE AND ACFT TRIMMED UP; THE AUTOPLT WAS RE-ENGAGED. THE ACFT IMMEDIATELY MADE A HARD R TURN AND SLIGHT PITCH UP. THE AUTOPLT WAS AGAIN DISCONNECTED AND LEFT OFF FOR THE REMAINDER OF THE FLT.