Something similar happened to my daughter in her '02 Bowtie. Turned out to be a clogged muffler. Dealer replaced with new, and presto....perfect fix.
She was driving down the interstate when the thing just quit, then when restarted would not go over 30-35 mph.
Do you get a "Check Engine" light? Any idiot lights? What does the tach do when you try to step on the peddle above 30 mph? We need more info besides "spit and spuder"
__________________ daughter owns '00 GT, bone stock, black on black, rag top
dad services and maintains, and researches mods | 
Mustang has no power after it gets to 30 MPH. Help ! 
