4.0 engine - no spark - won't start

I have a 98 4.0 ranger that fails to start sometimes. I have determined that I am not getting any spark to the plugs when the truck fails to start. The problem only occurs when the engine has been running for a while, then shut off for 15 or so minutes. It cranks but will not start. I open the hood and as soon as cools enough it starts right up. Any ideas as to the fix?