Another common reason for this trouble is needle in the wrong way. Cleaning out hook, race, bobbin and feed dog area is important too. You probably already have had these parts out, but if you haven't; take the throat and bobbin plate off, bobbin case, race and hook as well, scrape every groove and corner clean with a tooth pick. The tricky thing with model 15 is not all of them have the needle inserted the same way. Make sure you have the corrent bobbin type.
Sometimes dust and fluff gathers and push things of sync, thorough cleaning helps a lot. I once thought I had cleaned, polished and oiled a 201 to top standard. The needle was however slighly off its' mark, and the reason turned out to be hard packed dust behind the needle clamp. There was a groove in the needle bar that needed more cleaning.
Model 15s are nice machines, I have to say I like motor and lights, even for minor jobs, it leaves both my hands to maneuver the fabric.
This model handles just about every type of thread and fabric you can fit under the presser bar with ease.