A folded jacket off the back of a tailgate is not the most secure of gun rests but you mentioned a good group with the last shots - was that group also low and to the left? I would suggest concentrating on my grouping first, with a better and more stable rest, to determine if it is the pistol or you that is shooting low and to the left - IIRC, I believe that groups low and left used to indicate that you are jerking the trigger - it doesn't take much.
Don't mess with the sights until you ascertain postive grouping. Bring a friend to shoot the pistol and see if it shoots to the same poa, then you will know if it the sights or you.