So you don't have a cat on it if i read that right? If do when it starts to clog it up this will happen but its over a slow period of time its gets slower and slower and loose power but its right when it starts up, it don't have to run 20 miles. When after 20 miles when you floor it to go faster but it doesn't does it try ot sound like it wants to or just fall flat and not rev any higher? At this point can you rev it in park and get good solid rev or does it sound it like it does not rev up good high and solid. Maybe fuel pump getting hot running slow loosing fuel pressure, coil getting hot loosing spark this not a good burn loosing power? Running too lean or too rich? Another way to test this what I was told when cold get a hair dryer and heat those parts like the coil, if you heat the coil and it does what its doing now then that could be the issue a coil getting hot, fuel pump to test it get a fuel pressur tester give it the 20 miles then compare reading s from cold to hot see what the pressure readings are, if they are lower it could be the fuel pump getting hot and trying to shut down.