I have a Unimog FLU419. It's been a great vehicle but when you lift off the throttle, the vehicle becomes unstable, wanting to swerve back and forth so it doesn't feel safe. When you apply throttle again it stabilizes. The more quickly you lift off the throttle, the more radical the swerving. It used to be very stable and it used to go over 50mph but now it only goes 40mph and has this swerving problem. The only change we've made is to remove the part of the fuel filter which appears to be a bypass of some sort. If anyone knows what is causing this I appreciate your input.