Car vibrates when driving

[Dottie L. Simar]

I have a 2005 50th Anniversary T-Bird. The car was running smooth. After an oil change, I noticed that the car was vibrating when driving, expecially on smooth roads. It's like the car is running rough or unbalanced tires. Ford had the car for three weeks trying to find the problem. They cleaned oil on a gasket, change parts in the exhaust system, bolted the engine, etc. The car vibration is better but I still feel it even though it is subtle. I went from Shell Gas 93 octane to BP 93 Octane. It's not bad enough for me to make a big issue about it but it still is there. Can anyone help or is this something I have to live with. Thanks