Ok, my son's truck has an issue... We initially thought the altenator was hosed, but now, we are not so sure... The truck dies periodically and will not start--nothing happens--no lights, nothing when the switch is turned on. The battery stays charged, so that pretty much rules out the altenator. We can take the selenoid and connect it to the battery, the truck turns over. That leads me to believe that the problem is with the ignition or a switch somewhere--fuel pump, something. Anyone have any ideas??

Battery cables/connections.

Do the headlights work? If they do I would look at the ignition switch and/or wiring to it.

check your body ground too from the battery negative to where it connects(could be the engine block). be careful grabbing the negative wire- if its a bad ground it will be extremely HOT!

ignition switch

do what John said & ck your battery connections, especially the posotive cable between the battery & the solenoid