Pip, in this case, it goes from Doust to Dousted then back to Doust.
In other parts of my tree, there is Dowst from the early 1700's becoming Doust in the mid 1700's.
Then there are also a few other versions, Dost, Doast, Doustie, Dousett, Dousette, Douste, Douse, Dousty.
The people of today still can't pronounce it properly and so, I can understand why there are so many versions from days gone by!