As someone who dabbled with the two-handed game a few years back and who still bowls no-thumb as a B-game, I have some experience here. If I understand what you're saying correctly, it sounds like you're losing the ball during the backswing or forward swing, correct?
If so, this often has more to do with hand positioning and the swing itself than it has to do with fit. After all, if your fingers are the only part of your hand in the ball, you really aren't going to be doing much actual gripping.
As such, your setup has a lot of importance here. As a two-hander, you really need to experiment with where you put your off (non-gripping) hand on the ball because that will influence your feel and ability to feel like the ball is secure on the way back and during the downswing. I'm going to link to a page that has some videos of Jason Belmonte working with me on various aspects of two-handed bowling. The second video from the bottom provides some tips on hand positioning.
http://gkanak.wixsite.com/wannabbowling/repairsAnother aspect of this is where your bowling elbow is going. If the elbow starts flying around, it can make it very easy to lose the ball on the way back or to get all out of whack on the way back through. In time, I found that I couldn't push the ball down the way that Belmonte does. I had to try to keep my right arm almost totally straight on the way back and through; that was because I didn't have nearly enough flexibility to keep both hands on the ball straight behind me like he did. The same can be said of Jesper Svennson.
https://www.youtube.com/watch?v=i_jCrh0WSPAHis arms are long, and he's very flexible, so he can keep both hands firmly on the bowling ball when his swing gets behind him. If you can't do that, and many can't, you'll have to figure out ways of keeping the ball secure. I just don't think that the grips necessarily have a whole lot to do with it.
