Ok your Golf/Jetta doesn't like long motorway trips? You have to stop every so often, only to find it's fine again in 5 mins! Two words for you: Carb icing! Your carb is receiving a steady supply of cold air, depsite the fact that your engine is hot. The intake to the air filter should have two hoses, one takes in cold air from the front of the car, the other from a cowl around the exhaust manifold. Make sure the cowl and the second hose are both present and correct. If so, pull the two small vacuum hoses from the side of the air filter and connect them. Now try the car like this. If this suddenly cures the problem, you must replace the switch inside the air filter.
First things first, get down to your local fast-fit place and get them to perform a "drop-test" on your battery. This is usually free of charge and will often be the entire cause of all your problems!!