1989 Chevy Celebrity car stalls

1989 Chevy Celebrity 2.8 mileage: 126,500. I have a 1989 Chevy Celebrity 2.8, Auto/Air, which I bought used with 126,500 miles on it. Personally, I don't have much background in cars -- I'm a computer person. But, if you can help me look for and test things, I'll at least be able to get it to a qualified mechanic for the right problem. It has a problem that when it's been traveling at higher speeds (50+mph), as it slows down, say to get off the highway, it goes into a complete stall. I can start the car again right away in Neutral, but when I engage into Drive (whether stopped or coasting), the engine stalls again right away. If I let the car sit for several hours, it becomes usable again. This problem does not always occur, but it does happen most of the time. For example, I had it out on the interstate at 75+MPH for 20-30 miles without a problem; but a few days later, after taking it through some stop and go city traffic for a about 20 minutes, I got it up to 45-50MPH for about 2 miles, and as I slowed down, it stalled again. Weather conditions were about the same both days. The car does not appear to have ever been in any major accident, but there is a small fender dent in the left front. All in all, the car runs very well. One technician (who didn't look at it) said that it might have something to do with a vacuum line and some diaphragm; he said to trace it off the distributor cap, but I am not able to locate it. Do you have any ideas?