Cannot engage Auto transmission in Ford Galaxy 1996?

I have an intermittant fault in that when the engine is running I cannot depress the swith on the side of the Automatic transmission lever when the brake pedal is depressed. Thus I cannot drive away. This has only recently started to happen and occasionally a tap on the fuse panel to the right of the steering wheel will clear it.

It is a Ford Galaxy Ghia X 1996 model with automatic transmission and 2.8i petrol engine.