I Have a 99 Ford Explorer XLS 4X4 OHV and it does the same thing. I discovered that it is the Idle Air Control Valve(IAC) located under the intake manifold. You can try cleaning it first to see if that solves your problem, if not you can get a new one for about $45 to $50. Removal and installation are very easy once you locate it and identify it. Hope this helps anyone dealing with this annoying problem.