ALT DEVIATION; HEARBACK PROBLEM.

Narrative

I WAS FLYING THE ACFT ON AUTOPLT AND THE COPLT WAS HANDLING THE RADIO. HE ACKNOWLEDGED SOME ATC INSTRUCTIONS AS I WAS INVOLVED IN SOME OTHER COCKPIT DUTIES. HE THEN RELAYED TO ME OUR CLRNC TO 180 DEG HDG AND DSCNT TO 6000'. I SELECTED THE HDG AND HE SET THE ALT IN THE FGS AND ARMED IT. AS WE DSNDED THROUGH APPROX 6700'; QUONSET APCH SAID TO US THAT WE WERE ONLY CLRED TO 8000' AND TO TURN RIGHT IMMEDIATELY. WE SAW TFC AT 11:30-12 O'CLOCK AS WE MADE THE TURN; BUT I CANNOT GIVE A GOOD ESTIMATE OF THE DISTANCE. IN THE ENSUING DISCUSSION WE WERE CLRED TO 6000' AND HANDED OFF TO NY APCH. CONTRIBUTING FACTORS: 1) I DIDN'T HEAR THE ORIGINAL CLRNC WITH SUFFICIENT CONCENTRATION AS I WAS VERY BUSY HANDLING THE ACFT (STILL NEW TO ME) AND DOING THE WORKLOAD REQUIRED IN A 2-CREW ACFT. 2) POSSIBLE FATIGUE ON THE PART OF BOTH MYSELF AND THE COPLT AS WE HAD BEEN ON DUTY APPROX 12 HRS; ALTHOUGH I FELT ALERT AT THE TIME. 3) INEXACT COM BY ALL CONCERNED. THIS IS A HEAVY REMINDER OF THE NEED FOR UNWAVERING DILIGENCE AND CONCENTRATION. BUT AT SOME POINT; THE CAPT HAS TO TRUST THAT THE OTHER CREW MEMBERS ARE PERFORMING CORRECTLY. THIS COPLT WAS FLAWLESS ALL DAY AND IS FAIRLY WELL EXPERIENCED IN THE MLG. THE SUPVR AT QUONSETT APCH CONFIRMED THAT THE TAPES SHOWED WE ACKNOWLEDGED 6000' WHEN CLRED TO 8000'. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: RPTR STATES CONGESTION IN AREA IS PART OF PROB. FREQUENT CHANGES OF HDGS AND NAVAIDS WHICH ARE UNEXPECTED. WOULD NOT BE BAD IF ANY CONSISTENCY SO COULD PLAN FOR DSCNT. TURNED IN ANY DIRECTION AT ANY TIME. FOR EXAMPLE IN THIS INSTANCE; BRADLEY WAS NEWLY COMMISSIONED VOR NOT EVEN ON CHARTS YET. CREW HAD TO SCRAMBLE TO LOCATE IT AS WELL AS OTHER UNEXPECTED NAVAIDS. SOME OF THE FACS USED ARE NOT ON THE APCH CHARTS. NO CLUE AS TO WHAT TO EXPECT ONCE IN DSCNT PHASE. SUPERVISING CTLR SAID SECOND PROB THAT NIGHT. FEELS 2 PERSON CREW IN BUSY ENVIRONMENT DOES NOT ALLOW A BACKUP FOR FLT CREW. PRESUMABLY TFC WAS AT 7000' AS MLG PASSED THROUGH ON WAY TO 6000'.