AN ENG VIBRATION (MALFUNCTIONING MAGNETO) CAUSED A SCHEDULED COMMUTER FLT TO DEV FROM RTE; AS WELL AS DEST.

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RTING WAS ELH DIRECT TYM DIRECT GGT DIRECT FLL. ENRTE FROM TYM TO GGT; A 20 MIN HOP; I NOTICED A 'MISS' SOUND BUT COULD NOT IDENT WHICH ENG. ALL PWR AND ENG INDICATION NORMAL. DID A MAGNETO CHK. ALL CHKED NORMAL. ON CLB FROM GGT; NOTICED 'MISS' AGAIN. ALL OTHER INDICATIONS WERE NORMAL. I ATTRIBUTED THIS TO A COMMON OCCURRENCE WHERE THE SPARK WILL 'JUMP' INTERMITTENTLY AT ALT. THIS IS NOT UNUSUAL; SO I DID NOT RETURN TO GGT. AS I WAS HANDED OVER TO NAS APCH; THE 'MISS' HAPPENED AGAIN. IT WAS MORE NOTICEABLE NOW. I CHKED THE MIXTURE AND BOOST (FUEL) PUMPS. ALL ENG INSTS READ NORMAL. ABOUT 5 MIN LATER; THERE WAS A DEFINITE CHANGE IN ENG SOUND AND THE PLANE STARTED A NOTICEABLE VIBRATION. ALL ENG INDICATIONS WERE STILL NORMAL; THERE WAS NO SURGE OR YAWING. I STILL COULD NOT IDENT WHICH ENG WAS HAVING PROBS. I ELECTED TO DIVERT TO NAS; WHICH WAS ONLY ABOUT 30 MI NOW AND 15 DEG TURN R. DUE TO MY INABILITY TO IDENT THE PROB AND THE VIBRATION; I REQUESTED A 'PRIORITY' LNDG FROM NAS. THEY CLRED ME #1 AND SENT OUT THE EMER TRUCKS. APCH AND LNDG WAS WITHOUT INCIDENT. AFTER CONTACT WITH OPS; ON THEIR REQUEST I DID ANOTHER RUN UP. THE L ENG NOW SHOWED A PROB WITH A MAGNETO. ACFT WAS LEFT GNDED IN NAS.