RPTR DSNDS 500 FT PAST HIS ASSIGNED ALT BEFORE CORRECTING BACK.

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DURING DSCNT FROM FL350 INTO SLN ARPT WE WERE CLRED TO FL200 AND ADVISED OF A KING AIR 12 O'CLOCK 12 MI CLBING TO FL190. AFTER A MIN OR SO OUR CLRNC WAS CHANGED TO FL220 AND THEN THE KING AIR WAS CLRED TO FL210. WE SET OUR ALT SELECTOR FOR THE AUTOPLT TO LEVEL AT FL220. WE WERE ADVISED THE KING AIR WAS NOW AT 10 O'CLOCK AND 5 MI. WE LOOKED AND ANNOUNCED TO CTR THAT WE HAD THE KING AIR IN SIGHT. AS I TURNED BACK INTO THE COCKPIT I NOTICED OUR ALT WAS 21500 FT. I ABRUPTLY PULLED THE NOSE UP AND CLBED BACK TO FL220 VERY RAPIDLY. IT TOOK MAYBE 5 SECONDS TO CLB BACK TO FL220. AS WE LEVELED AT FL220 CTR ASKED OUR ALT. WE RPTED THAT WE WERE LEVEL AT FL220. (WE WERE WHEN HE ASKED.) WE CHKED OUR EQUIP AND THE ALT SELECTOR WAS INDEED SET AT FL220. HOWEVER; THE AUTOPLT ALT SELECTOR WAS NO LONGER ARMED. BOTH PLTS HAD OBSERVED IT BEING ARMED BUT NOW IT WAS NOT. (NO EXPLANATION.) WE WERE SUBSEQUENTLY CLRED TO LOWER ALTS. BEFORE BEING RELEASED TO SLN TWR CTR ASKED IF WE PREVIOUSLY COPIED OUR CLRNC TO FL220 AND NOT FL200 AND WE ANSWERED 'YES.' HE THEN ASKED; 'AND YOU DID NOT PASS BELOW FL220 CORRECT.' WE ANSWERED THAT WE HAD NOT PASSED BELOW FL220. THERE WAS NO MORE CONVERSATION ABOUT IT. WE DID IN FACT DSND TO 21500 FT AND RETURN TO FL220. APPARENTLY SOMETHING WAS BUMPED IN THE COCKPIT TO KNOCK THE AUTOPLT ALT SELECT OFF LINE. IT WORKED PROPERLY BEFORE AND AFTER. WE WILL IN THE FUTURE MONITOR OUR ALT AND AUTOPLT MORE CLOSELY. THE KING AIR AT 10 O'CLOCK AND 5 MI WAS NOT A THREAT. I COULD HAVE STAYED IN THE COCKPIT AND LET THE COPLT LOOK FOR THE TFC INSTEAD OF BOTH OF US LOOKING FOR TFC.