I had the same exact problem. I decided it was because the quilt wasn't feeding evenly because of the weight. I got a pair of quilting gloves (or actually mine are gardening gloves) and I couldn't believe the difference. It just helps so much more to maneuver the quilt. The stitches were mostly normal after using the gloves. There are still a few places that got hung up and the stitches are small. But, it's a big improvement. I bought gardening gloves at Lowes for $2.50. They have the rubber dots on the fingers and palms. I think regular quilting gloves are lighter weight but the gardening gloves work just fine.
Hope this helps you. I know how frustrating it is when things don't go right.