It sits there for a few seconds doesn't it? Part of the cruise brain has a circuit that if there is too much difference between the road speed and the set speed, it gives up and kicks out. If you try to move the set point too far to quickly, it will probably kick out. And like you said, if you have a problem where the throttle is not moving correctly and the truck is not responding by the speed increasing, it will kick out.
I notice it mostly when pulling a load and hitting a mountain. As the truck goes slower and slower, the cruise will finally kick out when the truck's road speed is too far away from where I had it set at.
