98 Ranger 4.0 intermittent starting problem

I have been having an intermittent problem with my 1998 4.0 Ranger. Sometimes it will not start, it cranks just fine, but will not run. Today it I could not get it started for about an half an hour. I tried putting a little gas in the air intake on top of the engine without any success. When it finally started, it just fired right up like there was no problem at all. Other times it starts in just a minute or so. It has been starting funny for a couple of months. Frequently it cranks for a second or two before it fires up. It used to start immediately. My mechanic says that unless it fails to start while he has it, he can only guess at what the problem my be. Anybody got any ideas on what the problem might be?

Update
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?