Do you have a picture of the "bunching?"
Without seeing one, it sounds to me as though you may be taking too much fabric out in the wrong place- for instance, if you take the seam in over your hips instead of just at the waist where it's needed, you may end up making it too narrow through the hips. When that happens, the shirt will ride up. You need to be careful to take in fabric only where it's needed, and not where it isn't. Vertical "fisheye" darts in the front and/or back might help with this also, and would help to distribute the difference around your body instead of taking it all out at the sides.