I have a 99 Expedition with a 5.4. On occassion when you try to start the vehicle it won't start, all it does is click the selenoid. I have replaced the selenoid and it started but it still does it on occassion. I have removed the started twice and had it bench tested and it worked fine. Put the starter back in and the vehicle starts but after awhile it does it again. Could it be a dead spot in the starter? A couple of times when it has done this after moving the selector in and out of park it has started, I don't know if it was just a coincidence or not. If it was the neutral safety switch I don't think the selenoid would be clicking when I try to start? Anyone have any suggestions?

 

I suspect there is a voltage drop between the battery,ignition switch and the starter solenoid.
One way to confirm this would be to connect a voltmeter to the small terminal on the starter solenoid/relay and observe the reading when turning the key to the start position. If you choose not to do this you could check and clean the connections from the battery positive terminal on up to the fuse panel. You will probably find one of them with some corrosion , which after cleaning will many times correct the problem.
It's always a good idea to do the same thing to the ground connections.