I had this same problem a cpl. of yrs ago. Went through about 8 needles til I finally turned to the rescue squad. This board. Once they told me to use heavy needles plus I focused more on my control my prob. was solved. I'm still not very good at FMQ but I do it.

I've honestly never used anything larger than an 80/12 needle for FMQ & regularly use a 70/10 needle with Aurifil 50/2 threads even when going over something many, many times -- like when working on a feather where I'm going up and down the stem as many as 6x. I do like using a Microtex needle because it is so sharp it easily fits into whatever tiny space there is (the same thing holds true for a Topstitch needle). I've used both of those, but never larger than an 80/12.
Here are tips that I've found tend to sort out the culprits: http://www.thriftyfun.com/tf/Repair/...g-Needles.html
Since it only happens on circles, I'd guess you are pulling a bit on the fabric as you try to move it around to form the circle. Either speeding up your machine or slowing down your hands should resolve the issue. Sometimes if my needle is struggling (because it's overdue for a change, or because I was lazy & tried to do dense FMQ with a quilting needle still in the machine), I try to overcompensate by pulling just a teeny bit more on the fabric. I realize this because certain muscles get tense at the same time that my needle starts having issues. By putting in a fresh needle & loosening up my "grip" on the fabric -- and taking breaks -- I get better results & the needles stay straighter.