Brother XL-4015, Top thread catching in bobbin casing?

I hope this makes sense to someone who can help. My machine regularly gets caught up. The top thread seems to be catching in the bobbin casing. I checked out the timing (from an answer to a previous question on this site) but the timing is fine, and the hook is working well. The thread just doesn't seem to be making it all the way around the bobbin casing once it's been hooked. It then begins doubling and trebbling with each stitch until the thread breaks. Is there any way to fix this?

Sandra K
September 2010