A B737-300 CREW HAD A TRACK DEV IN ZBW CLASS E AIRSPACE.

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THE AUTOPLT WAS ENGAGED IN LNAV AND VERT SPD MODE. WE HAD BEEN CLRED DIRECT TO EEN; THEN MHT; AND WE WERE ALSO INSTRUCTED TO CROSS 25 MI W OF MHT AT 9000 FT. AS DIRECTED IN OUR NEW PROCS; WE WERE NAVING VIA LNAV IN A -300; SO BOTH OF OUR HSI SELECTOR SWITCHES WERE IN NAV. I DO NOT RECALL WHAT WAS SET IN OUR MANUAL VOR WINDOWS. BASED ON MY PREVIOUS HABITS; IM FAIRLY CERTAIN I HAD THE LOC FREQ FOR RWY 17 AT MHT SET IN MINE; SINCE THATS THE APCH I WAS PLANNING. PASSING FL190 ON PROFILE; THE ACFT BEGAN A R-HAND TURN USING 15-20 DEGS OF BANK. AT THE TIME; I THOUGHT THAT WAS ODD; BUT I WAS MAINLY CONCENTRATING ON CALLING FOR THE DSCNT CHKLIST. AS THE TURN CONTINUED AND THE BANK INCREASED; I QUIT WORRYING ABOUT THE CHKLIST AND DEVOTED MY FULL ATTN TO WHERE THE ACFT WAS GOING. BASED ON THE WAY WE WERE APCHING EEN; I HAD EXPECTED A SLIGHT R TURN. HOWEVER; THE EXTENT OF THE TURN BEING CONDUCTED DIDNT SEEM RIGHT. I CHKED THE SCOREBOARD AND CONFIRMED LNAV WAS STILL ENGAGED -- IT WAS. I THEN LOOKED AT THE LEGS PAGE -- STILL SHOWED EEN-MHT ALTHOUGH THE HDGS WERE DIFFERENT THAN WHERE WE WERE TURNING. I QUICKLY TRIED TO XCHK RAW DATA; BUT WE DIDNT HAVE EEN IN EITHER WINDOW; SO IT TOOK A MOMENT TO FIND THE FREQ AND DIAL IT IN. AS I WAS DOING ALL THIS; I DISCONNECTED THE AUTOPLT AND BEGAN A L-HAND TURN BACK TO OUR ORIGINAL COURSE. (I WOULD ESTIMATE THE ACFT HAD TURNED ABOUT 45 DEGS TO THE R BY THE TIME I DISCONNECTED THE AUTOPLT.) DURING THE L-HAND TURN; I LOOKED BACK DOWN AT THE FMS AND NOTICED IT WAS BLANK WITH FMC FAIL DISPLAYED. ABOUT THIS TIME; ZBW BEGAN INQUIRING ABOUT WHERE WE WERE GOING. BOSTON GAVE US A CLRNC TO TURN L TO 020 DEGS AND TO MAINTAIN 16000 FT. AT THIS TIME; WE WERE ALREADY BELOW 16000 FT AND THE CAPT ASKED IF THEY WANTED US TO CLB BACK UP. BOSTON SAID AFFIRMATIVE AND SO I CLBED THE ACFT AND LEVELED AT 16000 FT AND ROLLED OUT ON THE 020 DEG HDG. I ASKED THE CAPT TO DO THE DSCNT CHKLIST TO GET OUR ALTIMETERS SET. DURING THIS TIME; BOSTON WAS ALSO GIVING CTL INSTRUCTIONS TO AT LEAST 1 OTHER ACFT WHO I PERCEIVED WE HAD BECOME IN CONFLICT WITH. AFTER A COUPLE MORE HDG AND ALT CHANGES; WE WERE GIVEN DIRECT MHT AGAIN. DURING ALL THE TALKING ON THE RADIO; WE WERE TRYING TO DO THE CHALLENGE AND RESPONSE CHKLIST. AT SOME POINT; ATC CALLED AND SAID THAT EVERYTHING WAS SORTED OUT AND THAT THERE HAD BEEN NO PROB. AS YOU MIGHT GUESS; THIS IS MY BEST RECOLLECTION OF THE EVENTS WHICH TRANSPIRED. IT ALL HAPPENED VERY QUICKLY AND AT A SIGNIFICANT POINT IN THE FLT WHEN WE WERE DSNDING QUICKLY AND TRANSITIONING THROUGH FL180. LESSONS LEARNED: WELL; FOR ONE THING; I DONT LIKE THE NEW LNAV GUIDANCE. HAD WE BEEN FLYING THIS SAME FLT 2 WKS AGO; I HAVE NO DOUBT THE PLT MONITORING WOULD HAVE HAD THE APPROPRIATE NAVAID TUNED UP AND WHEN THE TURN STARTED; I COULD HAVE KNOWN IMMEDIATELY THAT SOMETHING WAS WRONG AND MADE THE APPROPRIATE CORRECTION RIGHT AWAY. I ALSO THINK THE NEW POLICY OF NOT HAVING SILENT CHKLISTS IS FLAWED AS WELL. ITS DARK AND IM TRYING TO HAND FLY THE ACFT TO AVOID A TFC CONFLICT AND IM HAVING TO READ AND RESPOND TO A NEW CHKLIST -- NOT GOOD. OF COURSE; I WAS FLYING AND ACCEPT FULL RESPONSIBILITY FOR MY ROLE IN THIS INCIDENT. MY CRITICISM OF THE NEW PROCS IS THEY LED ME DOWN THE PRIMROSE PATH TO A STATE OF REDUCED SITUATIONAL AWARENESS. MY CRITICISM ALSO APPLIES TO THE NEW CLB CHK. DURING A VERY BUSY PHASE OF FLT; WE HAVE TO DIVERT OUR ATTN FROM FLYING THE ACFT TO PROVIDE AURAL RESPONSES TO ANOTHER GUY READING A CHKLIST. SUPPLEMENTAL INFO FROM ACN 615900: THINKING BACK ON IT; I THINK THE NEW PROCS SET US UP FOR THAT CONFLICT. FIRST WE WERE BOTH IN NAV; WHICH I DISAGREE WITH. IN THE -300 I THINK THE PLT MONITORING NEEDS TO MONITOR THE PROGRESS OF THE FLT. THATS TOUGH TO DO ON THE DSCNT PAGE WITH BOTH IN NAV. SECONDLY; WE WERE BOTH OUT OF THE LOOP INITIALLY BECAUSE WE WERE CHALLENGING AND RESPONDING TO THE DSCNT CHKLIST. THEREFORE; BOTH PLTS WERE DISTR FROM FLYING RATHER THAN 1 PLT. I THINK THAT CHKLIST AND THE CLBCHKLIST NEED TO BE ADJUSTED; OR CHANGED TO SILENT COMPLETION. WE HAD AN FMC ABNORMALITY THAT NEEDED TO BE THOUGHT THROUGH TO COMPLETION. THAT IS A LOT OF DISTRS DURING A SHORT PERIOD OF TIME. SOME OF IT WAS SELF-IMPOSED DUE TO THE NEW PROCS.