I purchased a 3D model from a turbosquid. It's a mailbox. All I'm trying to do is animate the door coming down. When I animate the door coming down on the Z axis, it looks fine and it starts and ends in the correct position. But in between the key frames it ends up doing a weird side rotation. I'm simply using auto key and then rotating only on the Z axis.
I think what you need is a linear keyframe rather than a bezier keyframe, which is what Max defaults to. Go into the keyframe editor, or I think you can just right click the keyframe just below your views, and change the keyframes to linear - that should solve the problem.