Ok, I have been working at this for weeks, and have come up with no answer. There is a single loud click coming from the relay near the battery fuse box under the hood on the firewall. I have had the starter, battery, and alternator checked. All are testing fine. It will start after a couple attempts, and sometimes will start on the first try. I am at a loss at to where to go next. Any help will be appreciated.

This is on a 1997 Ford F150, V8 4.6.