If I shoot my 30.06 15" Encore and shoot without touching the barrel with my support hand it shoots dead-on. If I LIGHTLY touch the top of my scope cap to stabilize the pistol with my support finger, I shoot 1-2" lower POI than before at 100yds. If the vertical stringing is not equipment related, hold pressure can dictate the vertical POI. Also if you have horizontal stringing, that is more to do with the pressure related to the grip hand. At least that is my experience.
You said the shots went back in place after you loosened your grip. That would most likely mean it is grip related and not an equipment problem.
If you are shooting for accuaracy (and who isn't) try to shoot with the same hold all the time. Get into the practice of staying with one type of hold.