I'll drive my durango for 2 hours of freeway driving and during the last 30 minutes of the drive the engine starts to surge and cough. It doesn't seem to matter what speed I'm driving. If I stop the durango and turn it off, when I turn it back on again, the engine will surge as if I have my foot on the gas pedal, but I don't. When I start back up again the engine is fine. It doesn't happen around town at all. I had a spark plug go bad a few years ago and it behaves similarly, but this isn't what this is. Any clues?