Rambling is what happens when your mouth leaves the house before your point finishes packing.

Speech can start before the thought has finished, and then the thought gets assembled in public. What comes out is the search itself, wrong turns included, and the listener has to stay for the whole process to discover where you were heading. Everyone does this, especially when nervous. It is a timing problem rather than a lack of clarity.

Before you start, decide on your last sentence. Say it first if you can, then explain. When you notice you are circling, stop and say plainly what you are trying to get at.