1994 Blazer - Runs bad after filling gas tank

My wife had been complaining that the truck was running rough. Had a couple of AAA tows. Also had the fuel pump changed by a local garage.
Also have had it to the Chevy dealer and they cant get it to run bad or just shut down (as it did once in a while).
So I used it for a few weeks. Real short distances to work. Finally I had to get gas. Filled the tank. Within half of a mile, the truck started to run like a dog!
Had to 2-foot-it to get home.
So now its narrowed down to where if I run it with less than a quarter tank of gas, its fine. Fill the tank - WHAM! Runs like garbage.
This is no coincidence. She recalls the truck also running bad after a fill up or just adding gas.
Anyone have an idea?
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