PIPER PA42 PLT HAS AN ALTDEV WHILE IN CRUISE FLT.

Narrative

I WAS FLYING S AT FL260 (-19C) IN IMC (HIGH OVCST CLOUDS) WITH INTERMITTENT 'LIGHT CHOP' (TURB) ON J79 BTWN MILIE INTXN AND OMN. ATC HAD JUST GIVEN ME ANOTHER AMENDMENT TO MY ROUTING WHICH WAS TO FLY DIRECT TO MLB AND THEN THE BLUFI 1 (STAR) TO OPF. I HAD ENTERED MLB IN MY GPS AND PUNCHED 'DIRECT' AND ADJUSTED THE HSI COURSE NEEDLE TO THE NEW COURSE FOR THE AUTOPLT (AUTOPLT WAS IN THE NAV MODE WITH ALT HOLD ENGAGED) TO FLY TO MLB. AFTER OBSERVING THE AIRPLANE TURNING TO THE NEW COURSE; I REACHED BEHIND THE COPLT SEAT TO PLACE THE H9 CHART BACK INTO THE CHART RACK (I ALREADY HAD H8 AND L19 OUT) AND PICKED UP THE FLORIDA TERMINAL PROCS FROM BTWN THE SEATS. I LOOKED UP THE BLUFI 1 STAR TO REVIEW IT AND ENSURE IT WAS PROPERLY ENTERED IN THE GPS. WHEN I LOOKED UP AT THE PANEL THE AIRPLANE WAS IN A DSNDING TURN TO THE R AND WAS ALREADY THROUGH 25700 FT. I IMMEDIATELY TOOK THE CTL WHEEL; LEVELED THE WINGS; ARRESTED THE DSCNT; AND BEGAN A CLBING TURN BACK ON COURSE. AS I WAS LEVELING AT FL260 AND JUST ABOUT BACK ON COURSE (AIRPLANE HAD DRIFTED ABOUT 30 DEGS TO THE R); ATC ASKED IF I WAS EXPERIENCING ANY DIFFICULTY. I EXPLAINED THE AUTOPLT HAD DISENGAGED WHILE I WAS REVIEWING THE BLUFI 1 ARR ON THE JUST AMENDED ROUTING. WHEN I GOT BACK ON COURSE AND LEVEL AT FL260 I RE-ENGAGED THE AUTOPLT AND IT FUNCTIONED PROPERLY THROUGHOUT THE REST OF THE FLT.