Gentlemen, I had same problem on my 3.7 roadster.
Remove spare wheel and you will see a rubber pipe going from the bottom near side of the fuel tank to the upper driver side fuel filler.
On my car this was S shaped and flattened on the two curves.
The pipe seemed to be a normal straight piece of rubber hose but too long for the job.
I just disconnected the pipe from the filler end and trimmed 25mm from the rubber pipe so that the pipe did not flatten out on the curves.
Hey presto - full speed fuel stops !!
