I have been on the starting strength routine for coming up on two months now. My upper body lifts have progressed some:

Bench - 110/115 to 160
Military Press - 75 to 105
Row - 115 to 160
Deadlifts - Up to about 200

However, one day I will squat 3x5 at 140 or 150, then the next day I will have trouble doing just 135.

I realized my form was starting to slack as I increased weight; i would start to push more with my tows and sway forward a bit. A coach at my school said I was turning it into a good-morning. So I am thinking of backing down on the weight. I try to do this:

Push with heals or at least have weight even over foot
Drive with chest instead of hips
Feet a little wider than shoulder width
Push knees out

My other problem is warming up. If I am doing working sets at 135, which should I warm up at and how many sets? It seams like my second and third working sets have better form and I think it is because I have problems warming up.