'98 Explorer Starts Hard - Sometimes

4.0L engine/auto transmission - 85,000 miles - no check engine light. Friend sez that when it happens, he can get it started and keep it running by holding down the gas pedal just a little. If he takes his foot off the pedal, the engine dies. He has to do this until it warms up - then it's fine. He sez it doesn't do it every time. He sez the first couple of times it happened, the front of the Explorer was facing up hill.

Any ideas?

Thanks...