I recently bought a 1999 35th Anniversary V6 Mustang for my step-son for his 17th birthday. It's in great shape considering it has 183,000 miles on it. But it's starting to have a problem. It has a vibration and stuttering that I don't notice at highway speeds but it's very noticable when just cruising around town. I just put new plugs and wires in car when we bought it since they had never been changed, it still had the factory original in the car.... the problem seemed to get better but not completely fixed. After about a week of driving the stuttering came back along with the check engine light. I took it to the shop and it's showing a misfire on cylinder one. They pulled the plug and it was cracked. They replaced the plug free of charge and it again seemed to get better but still not completely gone. Now after about a week I'm back to having the same problem again.... Anybody have any ideas? What could be causing this over and over again? And why is the problem never completely going away?