What was found
ON TAXI OUT THE TRANSPONDER RESET ITSELF TO 0000. PUT ASSIGNED CODE BACK IN. DURING CLIMBOUT ATC REPORTED A 600' DISCREPENCE BETWEEN OUR ALTITUDE AND THER RADAR, SWITCHED TRANSPONDERS AND THAT DID NOT HELP. SHUT OFF THEN ON, THEN IT WORKED. REMOVER AND REPLACED ATC/TCAS CONTROL PANEL IAW AMM 34-53-02-4, SYSTEM OPS CHECKED GOOD.