1992 Buick Park Avenue Engine stops when driven

The car is a great running car, except for here of late it's been stopping for no reason, usually after it's been running for a while and usually after you pull out or slow down for a light. The first sign is the speedometer goes past 100 or to 0, after that the engine stops and the check engine light goes on.
if you put the car in park it will starts right up. Of course when we take it to the garage it runs fine, and the mechanic tells us he will not be able to diagnose the problem with out replacing parts at random.