You're right Cathy, it can be direction and speed both. Sorry, I should have said that - I blame jet lag.
I did find I had some thread breakage on my domestic when I FMQd that was resolved with a larger needle.
Even some of the purpose built LAs object to certain directions (back and left most often) - I've never had a formal complaint lodged but I have had others tell me about it. I do use a larger needle than strictly needed
( I was thinking more about it, and I -think- it's a 4.0 in her right now - it's what she came with, and I'm roughly 2500mi from her right now so I can't go look. 
You can bet I have an itch to quilt or sew right now something fierce!) though in some cases, so that may be helping me, as you say.