There is definetly something buggy about that stretch of road. You get the same effect, if you just add a waypoint from the start to Davle, and set the speed to limited. Although I added a second waypoint close to the area where it swerves, and that appeared to fix it.
I have noticed a similar thing when you have an AI stood close to, but not on the road, the vehicle tends to swerve to avoid him. So even if you fixed the route it could still happen during the game.
But to be honest, I'm not sure thats the best way to move a bus from A to B. A pause of 0.001 seconds does not give your bus much time to get there?
I take it you dont want to use regular waypoints?