re: Taking a car to Firestone rather than the dealership.....

Firestone and all the other tire stores are just that, tire stores. When you need to see a doctor, do you go see an accountant?

Take your vehicle to a real mechanic shop. The dealership has all the latest up to date diagnostic equipment.

BTW, what is the year model and make of your vehicle?

Could be a small issue like a leaky vacuum hose