1990 ford bronco 351 windsor engine, e40d at transmission. Drove on hwy at hwy speed for 2.5 hrs, turned off and engine started to sputter and pop with very low idle rpm. Shortly idle went to 0 rpm and engine stopped runing, 1 hour afterwards engine started up and ran normall next day also. What is problem?

I have an 89 with a 351w. Mine did the same thing. Sounds like its vapor locking. After you let it sit for an hour or so it should start up but you should take the EGR valve off and either replace it or clean it with carb. cleaner. That should eleviate the issue. Carbon builds up in the valve and shuts off the ability for recerculation of exhaust fumes causing the vapor lock.

You'll have to check for spark at the plugs and fuel pressure next time it quits.
Could be ignition module, a relay, fuel vapor lock, or a list of other things.