1988 Oldsmobile Cutlass Wagon hesitates and stalls

1988 Oldsmobile Cutlass Wagon mileage: 210,000. This car has been a terrific car until the last week. Yesterday, it started hesitating (all the time) and cutting off at lower speeds. There is no warning, the engine just stops. It almost always restarts on the first attempt, but sometimes it takes a couple of restarts to catch. I have replaced the fuel filter (back by the left rear tire) and added fuel treatment (STP). Over 45 mph the problem is almost completely non-existent. Below 45 MPH, I can feel the car 'hiccupping' as I drive it. Can you point me in the most likely direction to start looking for the problem?