I agree with Danny. I spend a lot of time shooting and tuning and would have to say that you must relax after you shot a few arrows and therefore changing your grip.....
Try what I do - maybe you will focas more. I go outside - pickup one arrow and shoot - ONCE..... usually in the morning or right after work. It forces you to concentrate on THAT PARTICULAR SHOT, and not a group...... It really does work. Try it for a week.. You still want to do your normal practice but do it and wait an hour before you start your regular practice session or do it in the morning right before you leave for work.... If you are constantly hitting off your mark every time for a week - then as mentioned above you may be making adjustments to change your point of impact when perhaps your sight needs a minor adjustment.....
Try it - remember one shot everymorning before work for a week..... and see what the results are - remember - ONLY ONE SHOT... just leave your bow ready to quickly grab - and ONE arrow (dont cheat) Ill be curious to see your results =)

Tom