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Considering the age of your truck, you may have a cracked hose or a bed seal somewhere. B4 you start the truck some morning, lift the hood and look around. Take a good wiff for gas fumes. If you don't smell anything, have someone turn the key to the run position. This will pressurise the fuel system. Now sniff. When everything is cold, old/cracked lines will tend to leak. When things get heated up, the leaks can sealup. Good luck.
Thanks guys, I will do both. If it was a really old truck, where I could replace points, check timing, etc., thats the first thing I'd do. Smells like bad air/fuel mixture
Just out of curiosity, check the vacuum line to the fuel pressure regualtor for the presence of gas. If it has a regulator going bad it would be present and the diaphragm may equalize and maintain pressure once it's run awhile. Just a shot in the dark, by the way.