Turning off rolling cache seems to have eliminated the problem.
I turned off rolling cache and deleted the rolling cache file. After 6 flights in different areas of the world, including a test flight I’ve flown where I repeatedly get the drops from 35 to 10-15 fps, I’ve not experienced a single instance of this symptom. I still have rolling cache off, so I’m going to do another test turning rolling cache on which will create a new cache file.