Maybe you should check your instrument air. Lack of signal when disconecting the wires (depending on the failure state of the valve) may allow enough pressure to build up to actuate the valve again.
I have seen on a GE DC300, a fault with the field applying SCR package loosing a phase, inturn causing a week feild (or maybe the corect feild only half the time). When this happened under heavy loading it caused a huge current spike in the amature circuit. In the case I worked on, it blew up in...