A PA32 HAS RECURRING XPONDER MODE C PROBS THAT ARE FINALLY RESOLVED BY A UNIT CHANGE.

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THE PURPOSE OF THE FLT WAS TO SUPPORT OUR 2 JET ACFT. THE FIRST LEG WAS VFR TO ZZZ TO DROP OFF PARTS AT FBO X FOR OUR LOCKHEED JETSTAR. LEG #2 WAS OVER TO ZZZ1 TO PICK UP BRAKES; TIRES AND WHEELS FOR OUR LEAR 24. I AM FO ON BOTH THESE ACFT AND AM WELL TRAINED. LEAVING ZZZ1 AT APPROX XA00; I CHKED IN WITH DEP AND WAS TOLD TO SQUAWK XXXX. RADAR CONTACT WAS MADE; BUT THE CTLR SAID I WAS SQUAWKING XXYX. MY XPONDER INDICATED XXXX. I MOVED THE FIRST NUMBER AND THE CTLR SAID IT WAS GOOD. THE REMAINDER OF THE FLT WAS FINE. LEG #2: AFTER TKOF; I COMPLIED WITH MY CLRNC AND IN THE R TURN; ZZZ2 TWR TOLD ME TO CONTACT ZZZ3 TWR. THE ZZZ3 CTLR TOLD ME TO CROSS MIDFIELD. N OF ZZZ3; THE CTLR SAID TO SQUAWK VFR. I LOOKED AT MY XPONDER AND IT WAS INDICATING 1200. I MOVED ALL OF THE NUMBERS TO THE R; THEN BACK TO THE L WHILE CONTACTING DEP BEFORE ENTERING THE CLASS C AIRSPACE AT ZZZ1. DEP WAS UNABLE TO RECEIVE MY XPONDER AT ALL. I WAS TOLD TO CONTACT ZZZ1 TWR. THE TWR AT ZZZ1 ALSO WAS UNABLE TO PICK UP MY XPONDER. MY CONCERN IS THAT THE XPONDER MAY HAVE BEEN INDICATING SOMETHING OTHER THAN 1200. ZZZ2 TWR DID NOT SAY ANYTHING; BUT ZZZ3 TWR SAID TO SQUAWK VFR. LEG #3: DEPARTING ZZZ1 TO ZZZ4; CLRNC TOLD ME THEY WERE UNABLE TO PROVIDE CLASS C SVC WITHOUT A XPONDER. I THEN REQUESTED A R DOWNWIND DEP; UNDER THE CLASS C; THEN CLBED TO 5500 FT MSL VFR TO ZZZ4. MAINT CHKED THE XPONDER; AND I FLEW THE AIRPLANE ON APR/MON/01 CHKING IN WITH DEP OVER ZZZ4. DEP COULD ONLY RECEIVE THE PRIMARY. WE HAVE THE AIRPLANE AT FBO Y INSTALLING A NEW XPONDER.