Stress certainly affects the quality of my quilting! So I would suspect stress as the culprit, especially if you are doing FMQ or SITD.
Do you have a walking foot? What I would suggest is pulling one of your UFO's to sandwich, then use your walking foot to create gently wavy lines across the quilt. This is a very easy technique and there is no "wrong" way to do it. Here is a link to some images of the possibilities:
https://images.search.yahoo.com/yhs/...ppfocus1_ma_cr
And a link to a tutorial (there are more online):
http://www.bijoulovelydesigns.com/20...-tutorial.html