well first i would make sure the PS motor is running and providing pressure to the system. Then see if there is a pressure sensor anywhere in the system, at the pump or somewhere in the lines, maybe at the orbital valve? verify the sensor is working.
some systems require pressure to stay running, if the sensor is bad it may think the steering is idle and times out and turns it off.

other things to consider is inputs like seat switch and directional switch or possibly park brake switch input... some systems will not activate unless it has input from either of those sources and if it is loosing that at some point the power steering may turn itself off.