ALTDEV -- EQUIP FAILURE.

Narrative

UNDER THE AFOREMENTIONED CONDITIONS; MY ACFT WAS TRAVELING ALONG THE VICTOR AIRWAY IN LEVEL FLT. THE ALTIMETER INDICATED LEVEL AT 8000 FT MSL. BEFORE TKOF; THE ALTIMETER WAS SET TO PROPER ALTIMETER SETTING; AT WHICH TIME IT INDICATED NO MEASURABLE DEV FROM THE KNOWN ARPT ELEVATION. IN FLT AN AIRLINE PLT RPTED HAVING A 'VISUAL' ON MY ACFT LEVEL AT HIS ALT OF 9000 FT MSL. ALSO; HIS TCASII RPTED ME AT 9000 FT. ATC ADVISED ME THAT MY MODE C WAS INDICATING 9000 FT. ATC ASKED ME TO VERIFY MY ALTIMETER SETTING AND ALT. THE PROPER SETTING WAS ALREADY SET IN THE ALTIMETER; AND THE ALTIMETER DISPLAYED 8000 FT MSL. I ADVISED ATC OF MY ALT AND ALSO THAT I HAD NO VISUAL OF THE AIRLINE JET. THE ONLY CONCLUSION I CAN DRAW IS THAT; SOMEHOW; SOME MOISTURE MAY HAVE ENTERED THE STATIC PORT THE PRIOR NIGHT. THE ARPT WAS HIT BY SEVERE TSTMS THE PRIOR NIGHT. THE OLD PA28 I WAS FLYING WAS NOT EQUIPPED WITH AN ALTERNATE STATIC AIR SOURCE (ASIDE FROM SMASHING THE VSI). THE PITOT-STATIC SYS CHK IS NOT DUE UNTIL JUN/XX/95; SO THE ACFT SYS SHOULD HAVE BEEN FUNCTIONING PROPERLY. NO IRREGULARITIES WERE FOUND DURING THE PRE-FLT INSPECTION. HUMAN FACTOR PLAYED NO PART IN THE INCIDENT AS I WAS WELL RESTED AND FIT FOR FLT. INCIDENTLY; ANOTHER PERSON WAS ON BOARD THE ACFT AND VERIFIED MY ALTIMETER TO DISPLAY LEVEL AT 8000 FT MSL. HE TOO DID NOT SEE THE AIRLINE JET DURING THE ENTIRE INCIDENT.