I recently bought a mustang with a custom tune already in it. I did get the sct tuner with the vehicle however, there is a little problem with third gear. The rpm goes up and down as i hold the throttle steady and only goes away after i change to the next gear (Automatic). There is no engine light on, I was told I would have to reflash my car with the same tune and it would fix it. Do I take the same steps as if I were installing a fresh tune? I want to make sure I get this right because Ive read that a small error can really F up the car.

 

Yes, just reinstall the tune like you are doing it fresh.

But if it is a problem in the tune, reflashing it may not change anything and you may have to get the tune redone (and a new charge since you aren't the original owner).