98 k1500 starting problem

My truck always starts when cold. When I drive it for a while (15 min+) shut it off and try to start it after about 15 min. sometimes it won't start. And I mean doesn't even turn over. nothing. this is very intermittent...battery is good, connections are good...truck has 105,000 miles on it. heard about the starter relay switch...where is that? any suggestions???

Starter relay is the solinoid (part of the stater itself) fords have a seperate relay. You have same symtoms my 68 SS396 El Camino had. You may find this unlikely but it's true. The battery cable (+) goes directly to solinoid , but the 5 or 6 inches that pass close to exhaust got so hot over time the copper lost it's conductive properties and turned into a tungsten type of resistor. I cut the 5 inch section out of cable and ohm tested the two peices (with the cut piece out of the picture) 0 hm's. with the test before I cut it was 800,000 ohms. I never would hav found it . A GM engineer lived across the street and had metioned it, I said (to myself) whats this guy on? Then tried for the hell of it.