Sick Pony

[Foxtrot X-Ray]

Does it do that just when it's hot out?
If so, it could be a vapor-locking problem.

From experience with an embarrasing non Mustang: Heat from the road "boils" the gas in the tank or lines. Fuel pump won't pull vapor. Runs fine once it's cooled down.

VERY fast way to kill a fuel pump on a water cooled veedub.
(D'oh!! I said it. *Hangs head in shame*)

I haven't worked with Mustangs much. Just the little I got done on my GT before I lost it. So I could be way off base with this idea....