1994 Other Jeep Models hesitation

My Jeep Grand Cherokee has been acting as though it was going to stall, (and actually has several times)RPM's go to 0 and I have to pump the gas until it kicks in again. If I'm on the highway it's very scary (and I drive 25 miles on a highway every day to work). I've had it checked at several places and they can never duplicate the problem. I've replaced the air filter at my last oil change and the last guy who checked it out adjusted the idle which really only makes the car shake when I'm at a stop. HELP!