PLT OF AN ATX CARGO ACFT FAILED TO MAINTAIN APCH PROC ALT DUE TO ERROR IN INTERRUPTING THE PROC AND A POSSIBLE FAULTY ALTIMETER.

Narrative

I WAS INBOUND ON THE LOC 15 TO MTN; AND WAS PAST THE FAF; I BELIEVE 12.4 DME AT 2500 FT. XING THIS FIX I HAD A STEP DOWN ALT OF 2000 FT UNTIL 6.6 DME. I DSNDED TO 2000 FT AND WAS TOLD BY APCH TO CONTACT TWR. AT 10 DME I CONTACTED TWR; WHO TOLD ME TO CLB TO 2500 FT AND CONTACT APCH. IN DOING SO APCH TOLD ME THEY SHOWED ME 500 FT LOW ON THE ALTIMETER. I WAS ALSO TOLD THAT THERE WAS A TWR OUT THERE THAT THEY NEEDED TO KEEP ME AWAY FROM. I ADVISED THEM I WAS AT 2000 FT AND CLBING. LOOKING AT THE COPLT'S SIDE I NOTICED A DIFFERENCE IN ALTS. I TAPPED ON MY ALTIMETER AND THE NEEDLE SUDDENLY JUMPED UP. UNKNOWN HOW/WHEN OR WHY THIS OCCURRED. I THOUGHT I HAD ICED UP THE PITOT TUBE; BUT THIS WAS NOT THE CASE. I RESET A CIRCUIT BREAKER WHILE BEING VECTORED FOR ANOTHER APCH; BUT I LATER LEARNED IT WAS NOT ASSOCIATED WITH THE PITOT. THE SECOND APCH WAS UNEVENTFUL. I COULD NOT REPRODUCE THE PROB; AND IT DID NOT SHOW UP USING ALTERNATE STATIC SOURCE. HOWEVER; ON MY LAST LEG GOING HOME; I DID GET SOME NEEDLE WAVERING. I ADVISED OUR AVIONICS MAINT PEOPLE AND THE PLT'S SIDE ALTIMETER WAS CHANGED AND THE LINES WERE DRAINED.