I think this might be a case of temperature drift. As the current starts flowing through the sensor its temperature starts slowly to rise, changing the response over time. The best way to check if this is the case would be to monitor its output with a voltmeter. in this case there's little that c an be done other than re-calibrating with he circuit in its warm state. I never had this problem with the allegro, but I generally leave the joystick plugged in even if not in use.