Could just be a core strength issue. Are you puffing a deep breath into your belly before squatting?
Also, I hate the word "core."
Oblivian, I'm doing a 3x5 version of stronglifts (due to time constraints), so I squat 3 times a week.
I've taken some of the advice here during my last workout on wednesday, and it was much better, but that might be due to the lowered weight. I'll make sure to keep the advice in mind as I go up in weight.
I'll try to get a video up of my squats sometime before my gym closes for a week for renovations.