2000 F150 4.6 73,000KM.
When I don't use the truck for a week or two, the engine runs rough and the Service Engine Soon light comes on. After I drive it daily for 4-5 days, it runs smooth again and the light goes out. This has happened many times andonly when the truck is not used for a week or so. What could be causing this?

 

Sounds like you need to check up on your spark plugs and wires. Especially if it's rainy during the 4-5 days when not used. Otherwise, I'd be checking to make sure you don't have water in the gastank. Try DryGas in the tank next time it sits.