G'day WallyJ,
I've noticed this AI behaviour if the other choppers are too close. Move them and their waypoint to about 100m apart from each other (Use the editor grid as a guide = 100m when viewed in close.) and see how that goes. I've got 3 choppers landing on an LZ fairly okay that way. (Working towards 10 for the LZ!) If they are too close or the flyin height is too low one will porpoise and go way up high and stay there, and not make the land waypoint. Guessing the AI is cycling through it's crash avoidance code. I am using a flyin height of 70 non-ASL.
Good luck!