When driving for about an hour or more at freeway speeds (any more than 45mph) my 2000 Focus will suddenly start to stutter and stumble and cut out until it is jerking back and forth violently. I can slow way down and nurse it along for a bit but will finally have to pull over and turn it off. If I let it sit for a few minutes, I can usually get going again but the same thing will happen evey exit or so (about every ten - 20 minutes.) It will drive fine until it does this the first time, but once it's started, it will keep cutting out and dying until I can park it and let it sit overnight. At city speeds it seems to drive just fine. It's only been on longer drives and faster speeds that it has problems. No lights have ever come on, and I took it in for a diagnostic, which shows nothing. I've replaced the fuel filter, had an oil change and fuel system cleaning. The spark plugs are fine and it never seems to get too hot. It idles normal. Three mechanics have had a look at it and can't figure it out. It's pretty unpredictable and they can't recreate it. Any ideas? Hopefully it's not a fuel pump since the car isn't worth any major repairs. What could I do to diagnose the problem?