it definitely sounds like weak hip drive. i'm dealing with the same damn problem right now. i had to reduce the weight quite a bit and am slowly working back up. i blame all the years of squatting to parallel or ignoring them altogether and only doing leg presses (thank you muscle mags!)

anyway, a few things that have helped me are:
1. Adidas weightlifting shoes- i wouldnt even think of training without these now.
2. lifting my toes slightly just as i start to drive out of the hole- it just seems to help me consciously avoid shifting the weight to my toes
3. Sticking a plastic milk crate under my ass- similar to doing a box squat but instead of resting at the bottom and then exploding up, i begin the drive upwards as soon my ass touches the crate. It is helping me put my "mind in my ass" so to speak, lol!
it is also helping me achieve consistent depth.
4.- consciously tightening my hams and gutes on the way down-this has helped insure tension at the bottom.

i've had the shoes for a while but have been implementing the other points only for the last few weeks. My squats have improved quite a bit already so i'm pleased

good luck!

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