A good rule of thumb is to get all clips into exactly the same format as your timeline (video & audio codec, sampling rate, frames per second, etc). If I were you, I’d take the problematic video clip into Quicktime Pro (if you still have it) or After Effects and render it out to the exact specs of your editing timeline.
Of course, you don’t HAVE to do this all the time, but you sure will have a lot fewer problems if you do…. I suggest you give that a try and see if you still have the problem with video lag.