I had similar problems with a GP100 until I reworked primer pockets on the new Winchester brass I was using. I did everything under the sun to the gun with no improvement until either using factory ammo or reworking primer pockets. The Winchester brass pockets were not parallel to the case head or square side to bottom causing the primer to seat on a small spot only. Single action hits slightly harder than double making double action more sensitive.
New Winchester Brass stinks.