! Can anyone help me as I am still having problems with my vehicle!

My vehicle fuctuates with the shuddering as there is no particular time when driving it that I can pinpoint as to why it could be doing that! I was told that it could possibly be the coils but not 100% sure that I would need to get a diagnostic done on the vehicle - but in saying that the car really needs to be playing up so that it could give the technicians an accurate reading of the cause, also there would need to be lights coming up on the dash which would also be a good indicator - So I booked my car in for a diagnostic with KMart Auto's - some codes did come up apparently which did indicate the coils in which they had re-set the code but also my thermostat was replaced on top of that! I was also told that if the vehicle was still playing up to bring it back - well the vehicle still has the same problem and yes it was still playing up! Can anyone help me as I am feeling that I can't trust anyone at the moment!