having the same prob with my 98 gt, i tried plug wires fuel filter pcv valve, spark plugs. then it stopped for a day, then it started again, so i took it to the dealer for a diag, they say oh we need to replace wires, when i just bought wires; ford racing wires no less. i told them screw you and paid them 100$ for the diag and left. diag said it found a foeled plug (misfire) on cylinder 3, so i left the dealership and it ran great for like a month, then just the other day its doing the same crap!!!!