I have an 88 Caravan 3.0 MFI with the same type of issues. It is not overheating, but after running highway speeds it will suddenly stop running. Still getting spark, fuel, and air. After it cools for 20 minutes or so it runs with no issues.
I can get it to idle (rough at times), and it will not throttle up.
Acts a lot like vapor lock, but no vapor in the fuel lines???
The family and I drove it ~ 100 miles yesterday (mostly highway), and it stopped when we were only 10 miles from home. It has done this now on 4 different occassions, and it will restart after a cool down period and run fine.
Any help is appericated.
Justin