[Al Skierkiewicz]

Bruno,
If you have wired the SC correctly, (thanks Bill for the great catch and picture) then what you describe is a typical bad crimp or loose connection type of failure. When the current demand is high (wheel loaded) then the series resistance of a bad connection limits the current supplied to the motor, i.e. low power. Check that all connections are good, tight and soldered/crimped. Check that the breaker feeding that branch circuit is of the correct type and that the SC is showing normal LED function for your data control.