1996 Chevy Blazer 4.3 V6 engine shuts off.

I have a Chevy Blazer 1996 with 249,000 miles that shuts off at every stop light. It shuts off right at acceleration and it seems to be missing around 50 miles an hour (it doesn't like this speed). It does fine under or below this speed and then shuts off once you stop at a light or press the gas after it's been idle. It doesn't do it as much during the winter months but it still doesn't like to accelerate (i.E. Getting on expressway). Also, I smell a hint of gas once it's turned off but it's not leaking anything that I can see. I have changed the EGR valve and injectors. That didn't work. So I had the O2 sensor changed. That didn't work either. Finally, a guy looked at it and he said the computer said it still needed an O2 sensor. He saw that mine was new and cleaned the throttle body and gave it a tune up. Also, He checked to see if anything was plugged up. There was nothing in the exhaust or converter (which is new too). He saw that it has a new fuel pump as well. So, he has ran out of answers for me and suggested I needed a new engine. However, I don't want to replace a new engine and then have the same problems. Can you give me and idea as to what might be causing it to shut off like this?