About a month ago I bought a used Sigma 28-70 2.8 Ex Aspherical. I dropped the lens onto carpet, luckily not attached to the camera. Now anytime I try to focus on infinity, the sky, or a featureless light-colored surface, autofocus stops working. I can get it working again by swithcing to manual focus and runing it through the range, then switching back to AF. I was relatively happy with the lens, and did not pay a whole lot, is it repairable, or should I start shopping again?