96F150ED

I have a96 F150 XL with 4.9L and5 speed man. my question is if i drive for two or three hours and stop engine when i try to restart after about five min it will run real rough for the first 20 to 30 seconds then run fine it feels like it is not getting gas. the truck will start fine on short trips and first start in morning. the only problem seems to be on long trips!!!!

Please help