MustangWorks.com : Ford Forums

Very possible. I've had the exact same experience with Bosch CRAPINum plugs. Granted, it was with an older 4-banger LX, but the point remains the same. Just because you paid more for them does not make them better spark plugs. Platinum is not what Ford originally had in mind for your GT, and even though Motorcraft may offer a platinum plug, it is NOT the same as the Bosch design. Bosch decided to use a hair-thin center electrode; Motorcraft (and virtually every other plug manufacturer/label) uses a copper center electrode with platinum tips, and this is the vital distinguishing factor.

You may not have the wrong plugs in your GT, but they are probably stealing some power and gas mileage. This is because the flame kernel from a Bosch Pt plug is not nearly as large as a copper or copper/Pt plug's; Champion found this out many years ago when the Plats started coming out. They're not as bad in a wasted-spark ignition system (aka coil pack ignition), but just plain work terribly in a TFI-IV system (yours), in my professional opinion. :D

Change your plugs to Autolite/Motorcraft (same maker), Champion, etc., as long as they are regular copper plugs. Don't fall for the splitfire or other gimmick plugs. They are a complete waste of money and time, and if they actually did work, wouldn't the automotive industry have picked up on it by now???