I googled your machine model and it's actually pretty similar to what I have, a Brother XR3420, on which I quilt too.
The other month I was having problems quilting and here's what I found worked for me. I used a 100/16 needle; I put my tension dial to '5'; (I was FMQ so I had my feed dogs lowered,) I used 30wt cotton quilting thread, and I focused on moving my quilt at a slow but steady pace. I also cleaned my bobbin case and under the feed dogs at every bobbin change; probably too often but I thought better safe than sorry! Funnily enough, the thing that really worked for me was that, instead of using the FMQ foot that came with my machine, I found my old FMQ foot from my previous Brother worked better. I think the spring on it was better and floated above the quilt "just right".
I found that the top stitch needle did not work for my machine at all, so each machine is differnt.
I hope you are able to fix the problems and don't need to take it in for a service! The last time I took a machine in, a singer, I got it back with the timing just as rotten. A waste of $65. That's when I said " forget it!" And bought my Brother
Please let us know how it goes; I so feel you pain with quilting issues!! Been there, done that all too often!
And welcome to to the Board!!!