You've had some good advice. Taking the machine in for her spa day is a good move.
Then you need to trouble shoot your relationship with your princess.
It may just be that you are becoming more confident with your FMQ, and haven't upped the settings to go along with that.
Shredded thread - is almost always a needle issue. Either the eye, or the actual needle size is too small for the thread or fabric. I usually use a 12 for regular sewing, but use a 14 for FMQ. Topstitch needles do have larger eyes.
Once you are moving the fabric quicker and more confidently, you also need to speed up the machine to match.
Make sure you're not speeding up going around curves - a common problem. If you are, you'll get eyelashes on the back. Try to keep a consistent speed.
Then there is the usual - could be a bad needle, could be a bad spool of thread, could be just the wrong thread for that fabric, could be the wrong needle for that fabric, etc, etc.
I find if I've left my FMQ for a while, I need a breaking in period again before things get humming. It's like my machine and I need a little romance to get going! LOL