This is what happens:
The cart is a long vehicle and I have the feeling that the navigation point of the cart is in the middle of the cart...
This happens if points are too close to eachother:
http://uploadpie.com/kAbd5
He starts wiggling a little in the beginning to try and hit the next point, suddenly he will wiggle to much and be unable to "touch" the last point, which results in never ending circles around that point, trying to reach it.
This is whats happening on corners:
http://uploadpie.com/fwHdt
He aims for the corner point but because the cart turns slowly and its moving, it misses the corner point and also can never reach the point, so he starts circling it and crashes into everything trying to reach it.
Possible fix for corners: If there is a sharp corner (less than 120°) coming up, make the cart STOP and rotate on the spot before moving to the next point.
http://uploadpie.com/sBgOO
Possible fix for straight lines, increase the interval between points by a lot => standard is 2000, I've made it work with 4000.
Any more questions, I can answer
*Also, when mounted on a cart, it would be greatly beneficial to just decide to go to the next point instead of trying to circle around the current point, depending on the direction your cart is looking at.