1989 Mercury Cougar stalls when I accelerate

1989 Mercury Cougar mileage: 135,000. When you drive away from a stop sign or slow down to make a turn it will sometimes die when you begin to accelerate. It acts as if you have shut the key off, no warning or sputtering. We have replaced the ignition module and throttle control sensor. It does not do it all the time so they are not able to get the car to act up when they put it on their computer system. The mechanic said only thing left to do is to start replacing parts, which could become very expensive. Do you have any ideas or answers? Thank you for your time.