98 Ford Starter Mystery

I got a 98 Ford which runs great. Most of the time the engine turns over on the first try, just like it always has. But over the past 2 months at random times, you insert the PATS key and you get nothing, as if the battery was completely dead or removed. Then if I wait a couple of hours or over night, the truck starts right up like normal. The battery has been checked out is in perfect condition. My friend/mechanic checked the starter to see if it was pulling enough amps and he confirmed it was. It has happened a dozen times now. The last time it happened I took it to his shop and he check everything again: batter, starter, wiring and couldn't find anything wrong. He did not replace anything however. He maybe did something unintensionally because I didn't have any problems for two weeks, but today it happened again.

Any theories?

R. Pinder
Boise Idaho

Does the security icon, the key, flash fast when it won't start? Mine, an '04, will do that occasionally and all I have to is pull the key out and try again.....Mike