GARMIN G1000 ALTIMETER SETTING CHANGED WITHOUT PLT INPUT; RESULTING IN ALT DEVIATION OF 1500 FT.

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ON AN IFR FLT PLAN I WAS GIVEN VECTORS AND ALTITUDES TO FLY BY ATC. UPON REACHING MY ASSIGNED ALT OF 4000 FT I WAS TOLD BY ATC TO CLB TO 4000 FT. I REPLIED THAT I WAS AT 4000 FT. ATC RESPONDED THAT THEY SHOWED ME AT 2500 FT. I RESPONDED THAT I WAS AT 4000 FT AND WAS NOT SURE WHAT THEY WANTED ME TO DO. THEY RESPONDED THAT I NEEDED TO CLB FOR TFC. AT THIS POINT I NOTICED THAT MY STANDBY READ 2500 FT. CONFUSED; I CANCELED MY IFR FLT PLAN SO AS NOT TO ENTER THE CLASS B AIRSPACE AND REQUESTED DIRECT TO ZZZ. WHEN I LANDED I FOUND THAT THE ALTIMETER SETTING ON THE G1000 WAS SET VERY HIGH WHICH GAVE ME THE INCORRECT ALTITUDE READING. I ASSUMED THAT I HAD SOMEHOW ACCIDENTLY ADJUSTED THIS IN FLT. ASSUMING THAT I HAD SOLVED THE STRANGE G1000 ALTIMETER ALTITUDE READING PROB I AGAIN FILED FOR IFR TO ZZZ1. I MONITORED THE ALTIMETER VIGILANTLY AND THIS TIME THERE WAS NO ERROR. I ASSUMED MY PROB WAS SOLVED. AFTER SPENDING 5 MINS ON THE GND TO PICK-UP A PAX I BEGAN MY RETURN FLT TO ZZZ. THE ATIS WAS THE SAME AS I LANDED SO I DID NOT CHANGE THE ALTIMETER SETTING. THIS RTE WOULD BE THROUGH THE SPECIAL FLT RULES AREA. I CHECKED EVERYTHING ON THE GND; FLEW THE PROC AS PLANNED AND ALL SEEMED TO BE WORKING FINE; HOWEVER DEPARTING THE SPECIAL FLT RULES AREA AND TURNING WBND TO ZZZ I NOTICED THAT MY G1000 ALTIMETER READING WAS AGAIN OFF FROM THE STANDBY ALTIMETER SETTING. THIS TIME THE G1000 ALTIMETER SETTING HAD CHANGED AND I HAD NOT TOUCHED ANYTHING. THE NEXT DAY; I FLEW THE AIRPLANE WITH MY WIFE; WHO IS ALSO A PLT; AND WE PAID CLOSE ATTENTION TO THE ALTIMETER SETTINGS ON THE G1000; AUTOPLT AND THE STANDBY INSTRUMENT. ON THE TRIP THERE WAS NO ABNORMAL READINGS. DURING THE RUN UP FOR THE RETURN TRIP TO ZZZ THE ALTIMETER SETTING ON THE G1000 JUMPED FROM 29.99 TO 30.65 BEFORE OUR EYES. DURING THE FLT AT AN ALTITUDE OF 5500 FT WE SAW THE G1000 ALTIMETER SETTING JUMP AGAIN FROM 29.99 TO 28.84 WHICH NOW MADE THE G1000 READ 4100 FT WHEN IN ACTUALITY WE WERE AT 5500 FT. ALSO DURING THIS FLT WE NOTICED THE BAROMETRIC SETTING FOR THE G1000 MOVED WHEN THE OBS MODE WAS SELECTED AND THE COURSE WAS DIALED IN. CAREFUL ATTENTION WAS PAID TO MAKE SURE THAT I WAS NOT MOVING BOTH KNOBS AS THEY ARE ON THE SAME KNOB.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR INDICATED THAT THE OBS (OMNI BEARING SELECTOR) KNOB AND ALTIMETER SETTING KNOB ARE MANIPULATED BY AN INNER AND OUTER KNOB ASSEMBLY. IT WAS DISCOVERED THAT TURNING THE INNER; OBS KNOB WOULD CAUSE A CHANGE IN THE BAROMETRIC SETTING. HOWEVER; THIS SETTING ALSO CHANGED WITHOUT KNOB MANIPULATION DURING THE ENG RUN UP. THE AFFECTED UNIT HAS BEEN REPLACED.