ALTDEV ALT OVERSHOT -- ACR CLBS ABOVE ITS ASSIGNED ALT WHEN THE ALT ALERTER IS MISSET.

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REFERENCED INCIDENT OCCURRED ON SECOND LEG OF FLT JFK- SLC-LAS; PREDEP CLRNC RECEIVED ON GND SLC WAS FFU2-MLF -- THEN AS FILED -- MAINTAIN 8000 FT. EXPECT REQUESTED ALT 10 MINS AFTER TKOF. FILED (REQUESTED) FLT LEVEL WAS FL350. DURING CLB; AT APPROX FL250 CLRED TO FL280; AN ACARS MESSAGE WAS RECEIVED FROM COMPANY DISPATCH REQUESTING REASON FOR DELAYED DEP (6 MINS) FROM GATE SLC. I TRANSFERRED ATC COMS TO FO AND CONTACTED DISPATCH TO RESOLVE QUESTION. I WAS OUT OF THE LOOP!! DURING THIS TIME; FURTHER CLB CLRNC WAS RECEIVED FROM ATC BY FO. AT THE CONCLUSION OF MY TALK WITH DISPATCH MY ATTN WAS CAUGHT BY THE ALT ALERT LIGHT. ALT AT TIME WAS NOTED TO BE 31400 FT; VERY UNUSUAL!! A QUICK GLANCE AT APFD ALT WINDOW SHOWED MY ALT AT 32100 FT. BIG PROB!! SIMULTANEOUSLY; I REQUESTED OUR ASSIGNED ALT FROM ATC (REPLY WAS FL310) DISENGAGED AUTOPLT; AND RETURNED ACFT TO FL310. ALTIMETER INDICATED 31600 FT AT TIME DSCNT WAS INITIATED. WE ARE BOTH (CAPT AND FO) UNCLR AS TO HOW AN ALT OF 32100 FT CAME TO BE SET IN APFD ALT WINDOW. WE FEEL THAT THE ALT SET KNOB WAS INADVERTENTLY TURNED; AFTER THE FO HAD SET IT AS A FOLLOW UP TO ATC CLRNC; AS HE TURNED HIS ATTN TO OTHER DUTIES. LESSON LEARNED/REINFORCED: DO NOT ALLOW ATTN TO PRIMARY TASKS AT HAND; PARTICULARLY DURING ANY PORTION OF MANEUVERING FLT (CLB TO CRUISE CERTAINLY BEING ONE) TO BE DIVERTED BY REQUESTS FOR OR OCCURRENCES OF SIGNIFICANTLY LESS IMPORT. THE WORKLOADS EXPERIENCED BY 2 MAN CREWS IN TRANSPORT CATEGORY ACFT ARE HIGH AT ALL TIMES. MY ATTN TO A RELATIVELY INSIGNIFICANT DISPATCH REQUEST TOOK ME OUT OF THE SAFETY AND COMPLIANCE LOOP AT A TIME WHEN MY ATTN SHOULD CERTAINLY HAVE BEEN FOCUSED ON THE PRIMARY TASK AT HAND -- CLB TO AND LEVEL AT ASSIGNED ALT!!