B737-200 CREW HAD ALT UNDERSHOOT IN ZNY CLASS E AIRSPACE.

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DID NOT CHANGE ALTIMETER FROM 29.92 TO LCL ALTIMETER (30.45) WHEN DSNDING BELOW FL180. THIS PLACED ACFT AT 12400 FT IN HOLDING PATTERN AT STEFE VICE 12000 FT ASSIGNED. BROUGHT TO PLT'S ATTN BY ATC. NO MENTION OF TFC CONFLICT BY ATC; NO INDICATION OF TFC CONFLICT ON TCASII. WHILE AT FL260; ASSIGNED TO CROSS SOMTO AT 13000 FT. DURING DSCNT; NEW INSTRUCTIONS GIVEN TO CROSS STEFE AT 12000 FT OR AS CLOSE AS POSSIBLE TO 12000 FT AND HOLD; AS PUBLISHED; AT STEFE. ALTHOUGH FO WAS FLYING; I; AS THE CAPT; FAILED TO NOTE THE ALTIMETER CHANGE GOING THROUGH FL180. CAPT'S ATTN WAS GIVEN TO NOTIFYING COMPANY AND PAX AT HOLD; FINDING STEFE ON THE ARR CHART AND ENSURING NAVAIDS WERE PROPERLY SET UP FOR THE HOLD. I; AS THE CAPT; SHOULD HAVE DONE A BETTER JOB OF PRIORITIZING ACTIONS. ATC NOTED THAT WE WERE 400 FT ABOVE ASSIGNED ALT (12400 FT). THERE WAS NO MENTION OF A TFC CONFLICT. THE FAULT RESTS SOLELY WITH THE COCKPIT CREW. SUPPLEMENTAL INFO FROM ACN 495299: I ASKED THE CAPT TO CALL ATC AND ASK IF WE WERE GOING TO HAVE TO CROSS THE SOMTO INTXN AT 13000 FT AS INDICATED ON THE STAR. OUR PRESENT POS/ALT WAS GOING TO MAKE THE XING DIFFICULT. FREQ CONGESTION MADE IT DIFFICULT TO GET AN ANSWER FROM ATC. THE FAILURE TO RESET THE ALTIMETERS WAS A RESULT OF TOO MANY THINGS HAPPENING AT ONE TIME. THE LATE DSCNT STARTED THE SIT AND THE SUBSEQUENT HOLDING INSTRUCTIONS AND THE BRIEFING ON HOW TO HOLD; FUEL; ETC; AS WELL AS TRYING TO MAKE A RAPID DSCNT; LED TO FORGETTING A RELATIVELY SIMPLE TASK OF RESETTING ALTIMETERS. THIS WAS A FLC ERROR DUE TO A HIGH WORKLOAD ENVIRONMENT.