Check your intake pipe for a crack or other leak. What you describe sounds just like my 93 GT. It was split underneath the pipe right in the middle, about a 2" long split. Couldn´t see it unless I bent it, which is what´s happening when you hit the gas, the engine torques, causing movement in the pipe, letting in unmetered air. For a quick fix, try Shoe Goo. Or consider getting an upgraded intake using an aluminum pipe.