Stumbling after 20 miles of driving

[mr48chev]

There is no possible way that you can have a pressure build up in that tank except when it is sitting in the sun and heat. You have one line out and no lines in.

If it still stumbles when you leave the cap off the sock on the pickup tube is probably full of dirt or trash.
And I don't care if it is a brand new out of the box vented cap, unless you have tested it it is still suspect. As I stated earlier, I went through three brand name caps that were even marked vented but none of them would vent. I finally got a winner with the fourth one off the same parts shelf with the same parts number.

One other thing it could be is that the inside of the hose between the line and the tank is collapsed inside and sucks in and blocks off the fuel.