Plymouth Sundance losing accelleration

My daughter has been experiencing a problem with her 1994 Plymouth Sundance. It has approximately 100,000 miles. She says as she tries to accellerate, it will suddenly slow down to 30 mph or less, and will not return to speed. Sometimes the car dies and then will not accellerate. If she presses on the gas, the car does something she calls "bucking". We took it to our mechanic and he drove it and could find nothing wrong, so he tuned it up. Two weeks later it was doing the same thing. We took it to our friends trusted mechanic. He also drove it, found nothing wrong and tuned it up. Now, we have spend close to $500. And it is still doing the same thing. This morning it lost power on the freeway and dropped to 35 mph. We're out of money for this little car. Any ideas?