I only change tension with my walking foot if it needs it, which most of the time it doesn't.
I FMQ with my darning foot on and my tension usually needs a bit of tweaking to get it right. I usually make a practice sandwich with the same batting and similar fabric and use it to practice and to adjust my tension before I start on my quilt. If I get the tension right and then it goes silly, try changing your needle as it is usually the culprit. I hope this helps