SMA LEAVES ASSIGNED ALT WHEN NAV LOST; THEN RETURNS TO ASSIGNED ON IFR FLT PLAN.

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HAD GOTTEN A COMPLETE WX BRIEFING; BOTH THROUGH DUATS AND FSS. PROBABILITY OF TSTMS LIKELY ENRTE. DEPARTED IFR FROM SCH AND REQUESTED DEV AROUND SOME LARGE BUILDUPS. THIS HEADED US INTO MOUNTAINOUS TERRITORY. MORE BUILDUPS LOOMED AHEAD; ATC ALLOWED US TO DEVIATE AROUND THEM. WE GOT A NEW ROUTING CLRED VIA MONTPELIER-V104-BERLIN DIRECT AUGUSTA. ON V104 BTWN MONTPELIER AND BERLIN; WE LOST CONTACT WITH ZBW. I WAS NERVOUS ABOUT THE PROX OF MT WASHINGTON; THE TWRS ON THAT PEAK REACHED ALMOST TO OUR ALT. TO MAKE THINGS WORSE; THE VORS BEGAN GIVING ERRATIC INDICATIONS. RECEIVING NO XMISSIONS FROM ATC; A POSSIBLE ERRONEOUS INDICATION ON THE VOR IN IMC WITH TURB AND RAINSHOWERS; AND WORRIED ABOUT MY LOCATION WITH RESPECT TO MT WASHINGTON; I ELECTED TO CLB 2000 FT MOMENTARILY AS I PASSED THROUGH THE LAST OF THE HVY BUILDUPS IN ORDER TO AVOID CFIT. THE MT WASHINGTON AREA IS NOTORIOUS FOR SEVERE DOWNDRAFTS AND TURB; AND THIS WAS DEFINITELY A FACTOR IN MAKING MY DECISION. I TRIED TO NOTIFY ATC OF MY DECISION TO CHANGE ALTS BY TRYING TO CONTACT OTHER APCH FACILITIES; TO NO AVAIL. AFTER I WAS PAST THE MT WASHINGTON RANGE; I QUICKLY RESUMED MY ASSIGNED ALT OF 7000 FT. SHORTLY; THEREAFTER; BOSTON REESTABLISHED CONTACT THROUGH ANOTHER AIRPLANE TO ME AND SOON I WAS BACK IN RANGE. I THINK THAT IF I WERE TO ENCOUNTER THIS SIT AGAIN; KNOWING I WAS FLYING TOWARDS RISING TERRAIN; I WOULD ASK MORE PROMPTLY FOR A HIGHER ALT. THIS MIGHT HAVE ALSO KEPT COMS OPEN BTWN ME AND ATC! CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATES THAT THE ACFT IS A RENTAL AND CONSEQUENTLY SHE WAS UNAWARE OF THE INTERMITTENT VOR PROBS OF WHICH SHE LEARNED LATER. HER KNOWLEDGE OF THE MOUNTAIN IS SUCH THAT SHE KNOWS OF SEVERE DOWN DRAFTS IN THE AREA AND WINDS AT THE TOP OF UP TO 50 MPH AT TIMES. RPTR HAS ABOUT 15 HRS OF ACTUAL AND RECEIVED HER INST RATING A FEW MONTHS AGO. ACFT IS A PIPER ARROW. RPTR WAS COUNSELED QUITE STRONGLY ABOUT MAINTAINING ASSIGNED ALT AND CONSIDERING MEA TO BE A SAFE ALT. CONCERN WAS THAT SHE MIGHT BE OFF THE AIRWAY AND THE ALT WOULD NOT BE SAFE ENOUGH. ANALYST THINKS THAT HER KNOWLEDGE OF THE AREA GAVE HER ADDITIONAL AND UNWARRANTED FEARS.