A PA28-161 DSNDS BEFORE XING THE KWANG INTXN WHEN HIS #2 VOR INDICATES THE SPECIFIED FIX TOO EARLY E OF SBA; CA.

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I WAS ON IFR FLT FROM SANTA MONICA TO SANTA BARBARA. ON THE VOR APCH TO RWY 25 AT SBA; I WAS INSTRUCTED TO CROSS KWANG INTXN AT 4000 FT AND CLRED FOR THE APCH. WHEN MY #2 NAV INDICATED WE WERE AT KWANG; I BEGAN TO DSND. SBA APCH WANTED TO KNOW WHY I STARTED TO DSND. I TOLD ATC WE HAD PASSED KWANG; BUT ATC SAID WE WERE JUST GETTING THERE AND SHOULD HAVE STILL BEEN AT 4000 FT. I TOLD ATC THAT THERE HAD BEEN OCCASIONAL PROBS WITH THE #2 NAV ON THIS RENTAL ACFT -- THE NEEDLE WOULD STICK OR BE SLUGGISH OR INACCURATE. IT HAD BEEN RPTED WKS EARLIER; BUT NUMEROUS PLTS HAD NOT RPTED ANY PROB AND I DO NOT BELIEVE IT WAS EVER REPAIRED. I PERFORMED A VOT CHK AT SMO BEFORE DEP AND THE VORS WERE OPERATIONAL; ALTHOUGH #2 APPEARED SOMEWHAT SLUGGISH BUT PERFECTLY USABLE. WE WERE THEN GIVEN VECTORS TO THE SBA FINAL APCH COURSE -- THE 099 DEG RADIAL OFF THE GAVIOTA VOR. EVEN WHEN TUNED IN AND DOUBLECHKED; ATC SAID WE WERE ABOUT 1 MI L OF COURSE. THE NAVS ON BOARD (#1 AND #2) INDICATED WE WERE RIGHT ON COURSE. ANYWAY; NOT SURE WHETHER THE PROB WAS CAUSED BY A FAULTY NAV ON THE PLANE; A PROB WITH ATC'S RADAR; OR A COMBINATION OF BOTH. FLT LANDED WITHOUT INCIDENT.