1990 Cutlass Cierra Stalling on Slow Down

I have a 1990 Cutlass Cierra which has just recently began to stall intermittently. This has only occurred on long drives. After driving for about 50 miles or so when I slow down to take an exit it will shake and stall. It has been fine driving around town. I took it to a mechanic and since they could not replicate the problem they didn't do anything. Any ideas on what this could be. Thanks a lot.

Welcome to AF. What happens when you restart it and put it in gear?

After it stalls I can usually turn it right back on. Once I had to wait a few minutes.