PA32 PILOT REPORTS ROUGH RUNNING ENGINE AT 6000 FEET AND RECEIVES VECTORS TO SAFE LANDING AT NEARBY ARPT.

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ON A MORNING IN JUL 08; AFTER AN UNEVENTFUL PREFLT AND RUN-UP; I TOOK OFF FROM ZZZ1 IN MY PIPER SARATOGA (PA32) ENRTE IFR TO ZZZ2. I LANDED AT ZZZ2 AND I EXPERIENCED NO PROBS WITH THE ACFT ENRTE. I TOPPED OFF AT ZZZ2. AFTER A SECOND UNEVENTFUL PREFLT AND RUN-UP; I LEFT ZZZ2 LATER IN THE DAY INTENDING TO FLY IFR TO ZZZ1. I HAD FILED FOR 6000 FT. I EXPERIENCED NO PROBS AND NOTED NO DISCREPANCIES DURING THE PREFLT; RUN-UP OR ON TKOF OR CLBOUT. AFTER I CLBED TO 6000 FT; I ENGAGED THE ALT HOLD. I NOTICED THAT THE ALT HOLD ON THE AUTOPLT WOULD NOT HOLD ALT. I SO ADVISED CTR; AND I ASKED PERMISSION TO DSND TO 4000 FT WHILE I ATTEMPTED TO DIAGNOSE THE PROB. THIS REQUEST WAS GRANTED. I DSNDED TO 4000 FT AND DURING THE DSCNT I BEGAN TO HEAR A KNOCK IN THE ENG AND THE ENG BEGAN TO RUN VERY ROUGHLY. WHEN I ARRIVED AT 4000 FT; I STILL HEARD THE KNOCK AND STILL EXPERIENCED THE ROUGHNESS AND I COULD NOT MAINTAIN AIRSPD OR ALT DESPITE GOING THROUGH THE EMER PROCS. I SO ADVISED CTR TO WHOM I HAD BEEN PASSED. SINCE I COULD NOT HOLD ALT AND HAD TO CONTINUE TO DSND TO MAINTAIN FLYABLE AIRSPD; I COULD NOT MAINTAIN THE MINIMUM ALT FOR THE SECTOR. I ADVISED CTR OF THE SERIOUS SITUATION AND I ASKED TO BE VECTORED TO THE CLOSEST ARPT. THE CTLR THEN VECTORED ME TO ZZZ WHERE I WAS SAFELY ABLE TO LAND WITHOUT INCIDENT. THE 'KNOCKING' AND ROUGHNESS CONTINUED THROUGHOUT THE REMAINDER OF THE FLT. I HAVE ASKED A MECH TO INVESTIGATE THE PROB.