ALTDEV ALT OVERSHOT.

Narrative

DEPARTED MCO; HANDED OVER TO DEP CTL AND ASSIGNED 12000 FT. CAPT FLYING ACFT; FO SET 12000 FT AND ARMED ALT ALERT SYS. CAPT VERIFIED 12000 FT SET AND ARMED IN DFGS. CREW ATTN WAS DIVERTED MOMENTARILY BY RADIO CALLS AND REQUIRED CABIN ANNOUNCEMENT. ACFT PASSED THROUGH 12000 AND AT 13000 FT CAPT NOTICED ALT IN WINDOW SET AT 13000 AND ALERT SYS NOT ARMED. CAPT SUBSEQUENTLY DISENGAGED AUTOPLT AND RECOVERED AT 13400 FT AND BEGAN DSCNT BACK TO 12000 FT. SHORTLY AFTER; ATC CALLED AND INFORMED CREW 12000 FT ASSIGNED. CREW VERIFIED 12000 AND NOTED ALT WITH HIM. ATC THEN IMMEDIATELY CLRED US TO FL230 AND CLB WAS INITIATED. BOTH MYSELF AND THE FO CONCUR THAT THE DFGS ALT CAPTURE MODE DISARMED AT SOME POINT AND ALT WINDOW DISPLAYED 13000 FT AFTER OUR VERIFICATION OF 12000 SET EARLIER. WITH OVER 1000 HRS IN TYPE; THIS IS NOT THE FIRST OCCURRENCE OF THIS TO HAPPEN. OCCASIONALLY THE ALT SET WHEEL WILL LINGER BTWN DETENTS AND SUBSEQUENTLY 'CLICK' IN TO THE INCORRECT ALT; THUS DISARMING THE ALERT SYS AND DISPLAYING INCORRECT ALT.