Provided everything else is fine with your gun.. i.e dry barrel, tight sights, swabbing & drying the barrel every-other-shot, I would then first reduce the powder to 80 grains.. if it' s good there -- then try 85.

Another suggestion is to change sabots. You want " magnum sabots" ... they hold up better with " hotter" 777. A few magnum brands are out there. When shopping, try to match yours as close as possible to the new ones. If yours are now green, you may end up with black ones. Not all sabots are colored equal.