Are you running spektrum or some other DSM system? If so try rebinding the receiver to the handset and redoing the setup routine on the speedo with all settings on the transmitter set to neutral. A capacitor in the receiver might help as well. On an off chance have you got your PT too close to the receiver maybe?
Also check to see if the capacitor on the speedo is still ok as a duff cap can cause this type of behaviour as well.
Might also be a bust sensor on the motor and the speedo reverting to sensorless. Have you tried the motor with the Novak speedo and the Novak motor with the Tekin speedo?
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