Has anyone had the problem with the 70-200 f4 IS lens not working if the IS is on? This is a new lens and I've only used it a few times, but the last two times when I try to take a picture the IS motor just runs and runs and the shutter won't release. I've tried different settings and distances from subject but it doesn't make a difference. Sometimes it works and sometimes it doesn't. I can turn the IS off and it works fine. I keep thinking it's something I'm doing, but I can't figure out what. Any advice would be greatly appreciated.