I assume by cut a circle that you mean profile a circle. What CAM software are you using to generate the g-code for your profile and pocket? I'm wondering if the oval isn't due to some lead in/outs...otherwise kind of strange that a G02 or similar would yield an oval sometimes but not others...are you running aggressively and maybe losing steps in only 1 weaker axis? What material are you cutting and with what?
We'll get to the bottom of this. Eventually,