88 Buick century stalling problem

My son just bought an 88 Buick century with a 2.5 liter 4 cyl. The car runs good however if you get on the highway and drive @ 60 mph for about ten minutes when you get off on your exit ramp and decelerate the car dies, this happens almost every time you drive on the highway.If you drive in the city it doesn't happen only when you drive on the highway.Has anyone else experienced this? Thanks in advance for your help.

Ok here is a follow up . My son brought the car to me and I replaced the crank position sensor and the fuel filter. then I test drove the car I drove the car approx. 9 miles on the interstate and everything seemed ok as i got off the exit ramp it felt like the car wasn't downshifting and was in too high of a gear . as i got to the bottom of the ramp the car stalled. it started right up but as soon as you put it in gear it dies.Could this be bad trans.? maybe bad throttle position sensor? After i let the car cool down I was able to get it home.