You said that you use a light grip...
My advice would be that you use a firmer grip. Because the barrel is quite high above the axis of your arm on a revolver, they have noticeable leverage that pushes the barrel upward as the bullet travels down the bore. Recoil begins before the bullets exits the barrel. This is why heavier bullets at lower velocity will strike higher than lighter bullets at higher velocity.
Grab the the gun like you mean it, and I bet your groups will move down noticeably.