Solution found. I tried all the above suggestions, with no success. Took the boat and radio to my LHS. After checking with other Tx/Rx and servos, they felt it was likely a variable in the Tx program. It turned out to be the ABS parameter had been activated. Once the default was set, the speed control - rudder problem disappeared. The boat had its successful maiden voyage in the town local pool. Thanks to all for your help.