Let me say thank you for all of the information. I have tried all of the suggestions. I mostly quilt on my machine so I am usually sewing cotton. I have the nesting problem all the time. I solve it in various ways but sometimes get frustrated because I don't think this should be happening.
My question is if the 15000 or the 8900 also do this. My dealer is wonderful on fixing machines but not as good with troubleshooting actual sewing issues. I have actually debated trading in my 12000.
[Rummaging through the bag of helpful tips, Jim finds this.]
Have your dealer check out the hook race of the machine with a magnifying glass. Small burrs can cause this type of problem. Also have the tension release checked to make sure it really does release. A simple adjustment can fix that if it's not releasing properly.