This is how it seems for me, too. When I pay attention to what I am doing, it seems as if I am thinking in complete sentences, but if I then try to write down the idea that I have just been thinking about, I find that it takes a lot of editing to turn it into coherent language. Furthermore, I cannot really say anything about what it is like to think when I am not paying attention to doing so, so maybe it is just how I explain my experience of thinking to myself.

Perhaps experiments using techniques such as PET scans might reveal if there is something more characteristically linguistic going on in those of us who feel we have an internal monologue.