my 92' 3.0 was doing something close. i would get a gas odor in the cab for a few seconds after a warm start up. what i found was some of my o-rings on the fuel injector was weeping a little. it was such a small amount that it would leave no visible puddle of gas. just a little on the manifold. when starting up cold the gas had already evaporated (like setting over-night) so no gas smell. after starting up and some driving the fuel system was primed and fully pressurized and the leaky oring would allow a little to escape after i shut the truck off, when i fired it back up and it had not set long enough to fully evaporate i would get the gas (not taco bell) smell in the cab for just a few seconds. i found this by pulling off the plenum and noticing a bit of a stain on the manifold and around the injector. re o-ringed all the injectors and it went away. another symptom of fuel system leaking down is that it takes a few extra cranks of the starter before she starts up when cold.
could be your issue or something close....let us know what you find.