I am probably still classified as a beginning quilter (quilting one year to date). I started out with rag quilts (wise decision) and am now doing what I call "regular" quilting. Having said this, please feel free to ignore my advice! I actually ran into the same problems you are having. In regards to pleats and tucks, it sounds like the quilt isn't being fed evenly --being pulled down by its weight. I would suggest you slow the machine speed down some and keep things as straight as possible. Most important: USE A WALKING FOOT! If you don't have one, buy one a.s.a.p. It will make an important difference. Now, I also had the exact same problems you are having with the threading gathering in the back, etc. I adjusted tension, etc., and the problem persisted. I finally took my machine to a repair person for a quick evaluation; fortunately, the shop owner didn't charge me. He saw the problem immediately. My problem? A BENT NEEDLE!!! So, that could be your problem. I also had the tension set too high; I set the tension back around 4. I still had a few problems, and I finally purchased good quality thread. I've been using Arifil (yes, it is costly) thread and have had not one problem since then. I hope these suggestions help. Best of luck to you, and don't give up!
A day patched with quilting seldom unravels.