i have a 1987 mustang gt at my garage and i am stumped. car starts good, idles good, runs good but if you mash the pedal in first gear it bucks like crazy and lacks pull. in second car runs perfect but does the same in third and fourth but is ok in fifth. i have changed the plugs and wires there is 75 lbs of pressure on the pump. i have changed map sensor and checked iac. i need help