I am having trouble performing a successful ILS landing. I let ATC tell me i am clear to land runway XX and expect the vectors for an ILS approach and at that time I switch AP to nav and hit the app switch. The problems i am having is that a lot of times the plane will try to line up, but will end up swaying back and forth trying to get in the center and I end up running off the runway. Or, when the plane actually does line up, sometimes it will land short of the runway. Any ideas how to fix these problems or what I may be doing wrong?