PA60-602P ACFT #1 NAV SYS BECAME ERRATIC WHILE RPTR WAS IN HOLDING PATTERN AND WHEN DEPARTING HOLDING RPTR NOTICED DISCREPANCY BTWN #1 AND #2 NAVS. TWR AND CTR ADVISED RPTR THAT HE WAS 10 MI OFF COURSE AND PROVIDED VECTORS FOR APCH.

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CLR NIGHT; IFR FLT PLAN FROM LANSING; MI; TO LEBANON; NH. LEB RPTING A LAYER AT 4100 FT AND WINDS VARIABLE AROUND 4 KTS. THERE WERE 2 ACFT AHEAD FOR THE APCH; THE FIRST (SMALL SINGLE ENG) FLEW THE VOR RWY 25 AND RPTED PICKING UP LIGHT/MODERATE RIME ON THE DSCNT. THE SECOND; A COMMUTER; FLEW THE ILS RWY 18 AND RPTED NEGLIGIBLE ICING AS HE DSNDED THROUGH THE THIN LAYER. WE CHOSE TO FLY THE ILS AND WERE CLRED TO BURGR TO HOLD AT 6000 FT UNTIL CLRED FOR THE APCH. WE HAD HAD SOME PROBS EARLIER THAT DAY WITH OUR #1 NAV AND HAD HAD IT LOOKED AT IN LANSING. IT SEEMED TO BE WORKING ON THE FLT BACK TO LEB BUT AS WE ENTERED THE HOLD; THE NAV BEGAN TO BE SOMEWHAT ERRATIC. WE WERE CLRED AFTER A VERY SHORT TIME TO BEGIN THE ILS AND WE LEFT THE HOLDING THINKING WE WERE FLYING N AWAY FROM BURGR TO INTERCEPT THE LOC AND TRACK IT OUTBOUND. I NOTED THAT WE WERE MOVING AWAY FROM BURGR BUT NO INTERCEPT OF THE LOC TRACK. I ALSO NOTED A DISCREPANCY BTWN MY #1 AND #2 NAVS AS WELL AS THE COMPASS AND GPS. I HALTED MY DSCNT AT 4100 FT AND NOTIFIED THE TWR WE WERE GOING MISSED. THE TWR ASKED WHETHER WE REALIZED THAT WE WERE APPROX 10 MI SE OF THE VOR. WE; OF COURSE; DID NOT. WE SWITCHED TO CTR AND GOT A VECTOR TO RETURN TO THE VOR USING OUR #2 NAV. WE CLBED TO 5000 FT; POPPED THE ICE WE HAD PICKED UP AND FLEW THE VOR RWY 25; LNDG WITHOUT FURTHER DIFFICULTY. IN RETROSPECT; IT IS CLR THAT THE #1 NAV WENT BAD SOON AFTER I ENTERED THE HOLD BUT IT DID NOT PICK IT UP UNTIL I HAD FLOWN SOME DISTANCE AWAY FROM BURGR. I HAD SET MY SECOND NAV TO THE ILS AS A CHK AND AS SOON AS I PICKED UP A DISCREPANCY THAT WAS IT FOR THE APCH. IT WAS PARTICULARLY STRANGE SINCE A LEAR 35 HAD BEEN LOST AT LEB ONLY A FEW DAYS BEFORE AND IT HAD LAST BEEN RPTED ON A SIMILAR TRACK SE OF THE ARPT PARALLELING THE ILS. THE NAV WAS CHKED AND IS NOW OK. THIS REINFORCES MY REQUIREMENT TO USE THE SECOND NAV TO CONFIRM THE TRACK OF THE FIRST. ANY DISCREPANCY IS CAUSE FOR GOING MISSED; GETTING HIGH AND SORTING IT OUT AWAY FROM THE GRANITE. BOTH THE TWR AND ZBW WERE EXTREMELY HELPFUL DURING BOTH APCHS. CTR; ESPECIALLY; WAS SENSITIVE TO THE POTENTIAL ICING PROB AND MOVED ME THROUGH THE LAYER QUICKLY.