Rob,
I gripped my bow exactly like you described for a number of years. My shooting was too on-again, off-again. I went to gripping my handle so that all my fingers make contact with it. I found that this grip controls the bow more consistently for me. It seemed that with just a thumb and index finger grip, at least for me, the bow had too much of an opportunity to torque or "float".
Try it. It may help.
Tom.