The auto gearbox is computer controlled and that computer is linked to the engine management computer. These days, if either of these computers detects a fault, it will put either the engine or the gearbox or both into something commonly called “limp home mode”. Where the car runs at reduced performance, but at least keeps going so you can get home or to a garage so you are not left stranded. This seems to be what is happening with your Kangoo.
I suspect an intermittent fault on a sensor or the wiring to it. Because it is intermittent, when you start the engine again everything is fine and the computer puts the car back into normal driving mode. The good news is that the computers between them should store information about faults that have occurred, and either a Renault dealer or an independent garage with the right equipment should be able to read those codes and identify the problem area.
Since the car is under three years old, it must be under warranty, and I assume you are taking it to a Renault dealer. If so, and they haven’t found the problem by now, I would be going to a different dealer.