first of all, check for dust bunnies in the bobbin case---have you ever cleaned that area? Second, unthread and rethread the machine and bobbin--could be that you have not caught the thread in the tensions areas of both. That happens all the time on my serger if I don't get the threading right in the loopers. Also make sure your bobbin is in the machine the correct way. That would cause this also.

Always thread your machine with the pressure foot up and hold the thread taught with your right hand and thread with your left. This ensures the thread goes into all the tension disks. When you get to the needle, lower your pressure foot.

You will know if the thread is not correct if you get birdnests on the bottom. If you bobbin is not correct, there will be funky stitches on the top. Hope this helps.

This happened to me, and it was just a lot of dust that had built up behing the bobbin case, and there was also a huge amount of dust build up under the throat plate, So if you check that out, clean if out with a "Q" tip, place a a drop of oil on the bottom area where your bobbin case sits, (sewing repair man told me that all machines should get a drop of oil, every so often, sewing wears out this area.

I also take out the needle, and press the presser foot down as far as it will go, and run it backwards as well, do not use the same thread on the top as in the bobbin, I have found that not all threads like each other.

Put the needle back into its position, rethread your machine, hold both bottom and top threads while stitching the first 3 stitches, place material under pressure foot and attempt to sew - stitch 3 stitches then carry on stitching on your fabric.

If this doesn't work, take to your repair man, but it did work on my Bernina.
Best Wishes

I have had the same problem, my problem was I was using the wrong thread that made the bobbin thread do that .. check and see what type of thread you are using ..

Get out your manual and check trouble shooting.