Has anyone had this problem? I bought a new Smart by Pfaff (not a cheap machine). I had a Queen top about half done when I got the new machine. I finshed that top and started a baby quilt. I had no problem pieceing the top but when I started quilting it the thread balls up on the bottom side. It might sew 6 foot ball up 6 inches then go back to working fine. This is the first time I have tried quilting (my 3rd top). If this keeps up I will just make tops. I have read on this board about thread I am using a cone of polyester thread from JA. I have changed needles, changed thread, cleaned and recleaned the bobin area, rethreaded several times. My daughter told me about pulling my bobin thread through to the top when I start. Done that. When it is acting up sometimes it will run slowly without my foot on the pedal. I am going to take it back to the shop but it is just a timing thing.

I would say if it is running without your foot on the pedal it is something that the dealer needs to figure out.

It's probably some easy thing like forgetting to lower the feed dogs or something. But since it is a new machine, take it in and have it checked. The dealer may be able to give you some special sewing instruction if it's not a mechanical thing.

When I had my Phaff I needed to use top quality thread.

Try a different thread on it. Sorry, but I think the thread you're using is very low quality.

Have the machine checked.
I have 2 Pfaffs love them both.
Have you checked tension? When threading your machine keep the presser foot up. Don't know why but this can make a difference with some machines.

I had my Mother's Kenmore after she passed away, and it kept doing that. Well, I didn't take it in to be checked and lo & behold the foot pedal caught fire(the wiring shorted out). It ended up costing me way more than it should have if I had taken the time to have it checked.
Now "When it doubt...Check it out"

I have two Pfaffs as well and they both "prefer" (lol) high quality thread. I use Presentia for piecing - never tried FMQ but SID all the time with it. I also have tried pre-wound bobbins in them with mixed results. They are great machines but are finicky. I would bring it in to be checked just in case.