I just realized the lense can't hold a focus that I get by zooming max into the subject and then zooming back out to the widest point. It seems I have to go to M99 to get good focus while there's still 1/4-1/5 left in the zoom range. I've just noticed this now, I've always been able to hold a focus across the entire zoom range with other cameras. Any ideas why this might be?

Yang,
I do not own this camera, but what you describe sounds like a back-focus calibration issue (assuming that you are using a manual focus mode).