ACFT CLIMBED 600' ABOVE ASSIGNED ALT.

Narrative

THIS SITUATION OCCURRED WHILE AT A CRUISE ALT OF FL260 AND ON THE SCURRY ARR FROM SHV INTO DFW. WE WERE FLYING AT .76 MACH IN SMOOTH AIR AND IN VMC CONDITIONS. I WAS FLYING THE MLG WITH THE #2 AUTOPLT ENGAGED. ALT HOLD WAS ANNUNCIATED ON BOTH FLT MODE ANNUNCIATORS. WHILE APPROX 80 MI E OF SCURRY; WE RECEIVED A CLRNC TO FL240 AT PLT'S DISCRETION. FL240 WAS DIALED INTO THE ALT REMINDER AND ALT ARM WAS ANNUNCIATED ON BOTH FMA'S. WHILE LOOKING OVER THE CHARTS IN PREPARATION FOR THE ARR; THE ACFT BEGAN A SLOW CLB (APPROX 300 FPM) W/O ANY PLT INPUT. I FIRST NOTICED THE DEVIATION WHEN THE ALTIMETER INDICATED 600' HIGH. I IMMEDIATELY DISCONNECTED THE AUTOPLT AND RETURNED THE ACFT TO FL260. WHILE LEVELING AT FL260; FT WORTH ASKED US TO CONFIRM OUR ALT. WE DID SO AND OFFERED A BRIEF EXPLANATION OF WHAT HAD HAPPENED. THE AUTOPLT WAS RE-ENGAGED AND WE BEGAN OUR DSCNT. WHILE ON THE ARR AT 11000'; AGAIN WITH ALT HOLD ANNUNCIATED; THE AUTOPLT STATED A SLOW UNCOMMANDED DSCNT OF APPROX 200 FPM. THIS TIME THE AUTOPLT CORRECTED ITSELF BEFORE GOING MORE THAN 100' LOW. THE ACFT HAD BEEN WRITTEN UP THE PREVIOUS DAY FOR THE SAME TYPE OF MALFUNCTION; AND MAINT ACTION HAD BEEN TAKEN. WE HAD ALSO FLOWN THE ACFT ON 5 PREVIOUS LEGS THIS SAME DAY WITH NO OTHER AUTOPLT PROBS. UPON ARR AT DFW; WE ENTERED A LOGBOOK WRITE UP AND BRIEFED THE OUTGOING CREW.