AFTER RECEIVING A DSCNT WITH A XING RESTR; AN A320 FLC BECAME DISTRACTED BY COPYING THE ATIS AND A FLT ATTENDANT'S REQUEST. WHEN IT WAS DISCOVERED THAT THE DSCNT POINT HAD BEEN PASSED; THE EFFORT TO MAKE THE XING RESTR REQUIRED A DSCNT SO RAPID THAT THE ACFT WAS NOT LEVELED AT THE ASSIGNED ALT. ALTDEV ALT OVERSHOT.

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WE WERE IN CRUISE FROM DTW TO LGA WHEN ATC GAVE US A CLRNC TO CROSS MARRC INTXN AT FL190. I ACCEPTED THE CLRNC; SET THE ALT IN THE FLIGHT CONTROL UNIT WINDOW; POINTED TO IT AND SAID TO THE CAPT 'MARRC AT FL190.' HE RESPONDED WITH CONCURRENCE. I PROCEEDED TO GET THE ATIS AT LGA AND COPY IT DOWN; AND AT THE SAME TIME OUR LEAD FLT ATTENDANT CAME IN AND WAS TALKING TO THE CAPT ABOUT NEEDED ITEMS IN LGA. I LOOKED UP AND NOTICED WE HAD PASSED OUR NORMAL DSCNT POINT; AND TOLD THE CAPT 'WE NEED TO START DOWN;' AT WHICH POINT HE BEGAN AN EXPEDITIOUS DSCNT TO TRY TO MEET THE RESTR AT MARRC. I CONTINUED COPYING THE ATIS; AND ALSO ASKED THE CAPT IF HE WOULD LIKE ME TO INFORM ATC THAT WE MAY NOT MAKE THE RESTR; HOWEVER HE DECLINED AND FELT THE RESTR COULD BE MADE. BECAUSE OF THE INCREASED RATE OF DSCNT; THE AUTOPLT DID NOT APPEAR THAT IT WAS GOING TO CAPTURE THE ALT VERY WELL; AT WHICH POINT THE CAPT DISENGAGED THE AUTOPLT AND ATTEMPTED A MANUAL LEVEL OFF. THE ACFT DSNDED BELOW OUR ASSIGNED ALT DURING THE LEVEL OFF MANEUVER. THE CAPT IMMEDIATELY CORRECTED BACK TO FL190. NO CONFLICT WAS ENCOUNTERED. SUPPLEMENTAL INFO FROM ACN 329000: ON FULL AUTOFLT; THE ACFT WAS STILL DSNDING AT ABOUT 5000 FPM. PASSING FL198 DISENGAGED AUTOPLT TO LEVEL OFF; BUT DIDN'T ACTUALLY LEVEL OFF UNTIL FL182 AND IMMEDIATELY CLBED BACK TO ASSIGNED ALT OF FL190.