Theoretically, if you've had your fit and pitches checked by two different shops, at least one of the two should've ensured that things were as they are supposed to be.
When you say that your hand is relaxed, what do you mean/how do you know? Let me ask you this. Put your bowling hand palm up. When you take your grip into the ball, what best describes the position of your thumb in the thumb hole? Is it bent inward toward your palm, pointing dead straight, or arched backwards away from your palm and pressing into the back of the thumb hole?
I ask because it's very, very easy to start gripping the ball without even realizing it. One of my pre-shot thoughts is to make sure I can feel my thumb pressing against the back of the thumb hole to guarantee that I'm not squeezing on it. Without that check, I find myself backsliding into squeezing, which throws off just about everything involved in my release. I would check that first.
Tension in your bowling arm can also cause muscle/grabbing. Mike Fagan is probably the best example of a totally free arm swing. Anthony Pepe is pretty good too. The less tension/muscle you involve in the swing itself, the less you're likely to impart at release. There are several drills you can use to practice freeing up your arm swing and release. I would hunt around on the net, view some videos, and take to the practice lanes.
Also, if you have video capability, consider shooting some vids of yourself and posting them here for us to see. Seeing what you're doing is more helpful than reading about it. In any event, good luck getting things ironed out, and stick with it; we need all the bowlers we can get!
