Our shower just shuts off by itself after 1 minute. When using the shower, the water runs for about 60 seconds and then just stops. The water is still on and will run in the bath if we turn the knob, but when we turn it back to the shower it's still stopped. When left alone for a while and the water is turned on again, it runs through the shower ahead, but again, lasts only about 60 seconds and shuts off. We tried draining all the water thinking the air vent had filled with water. This helped a little for the first time and then the problem started up again. Any ideas?