Default 747 as AI Traffic

Don't know if it is me or what, but i have been unable to use the default 747 as an AI plane. It will float 2/3rds of the way down the runway at 200ft. and then either start to declare a missed approch and then plunge down or just descend so steep that it disappears on hitting the runway (I assume it exceeds the crash limit). I thought it was a 3rd party texture I was using, but I tried it with the default textures also. I've tried swapping the air file (such as the POSKEY one with no better results) Any suggestions? Fitz