Ai flight Problem

Hi

I've created some custom ai flights using the ai flight planner utility. I'm using add on aircraft and add on scenery. I seem to have a problem where traffic that I have created will take off and just continue to fly on the runway heading without turning or anything. It continues to fly on this heading for ages and ages (well past the time that it was due to arrive at its destination) and if i go into a menu and edit something or look at the map when I come back the traffic has jumped to it's destination airport. I'm using the latest version of ai flight planner and fsx acc. 

Any help would be much appreciated.

Cheers

85tromby