When this happens, the only thing I can do is get a big piece of paper and write down the pinging thoughts. Make a list. I keep it with me all day catching little scattered fragments.
Then I carve out time--a big block--where I start at the top and work all the way through to the bottom.
For me, scattered thoughts are usually about too much to do--so much that I lose my ability to prioritize. When that happens, the only thing left is to take it bird, by bird.