Topp Dogg, I'm not a coach - just an average league bowler. But I've had the same problem and have been told by a coach that not a tight thumb hole, but rather a snug fitting thumb hole works fine - just snug enough to allow you to relax your grip during the downswing (also helps provide a free arm swing because you're not grabbing the ball).
Staying behind the ball all the way until the thumb starts to come out also helps because if you come around the side of the ball too soon, gravity pulls the ball off your thumb too early and you will grab it. Staying behind the ball during the downswing, and letting the ball fall, all the way until the release will allow gravity to keep the ball on your hand with a relaxed grip. (it's like swinging a bucket of water instead of a bowling ball - if you let your arm fall back and forth, gravity keeps the water in the bucket, but if you turn the bucket to the side the water will fall out).
A relaxed grip, a relaxed arm-swing, and staying behind the ball I believe is the key. It's helped me.
Also, psychologically, its helpful to not tell yourself "don't grab the release" because then your focus is on grabbing the release. It's better to think something positive like "relax the thumb." The only pressure at the release should be applied by the middle and ring fingers.
