Thanks for all the thoughtful and helpful replies. I am using a 5 micron charcoal filter as well as a 50 mesh screen at the city water inlet.
Almot, I think you have the answer to my question. I once had a sailboat with a diesel auxiliary engine. Going up the coast of Washington, the engine began to slow down, then it would speed up again and cycled like that throughout the night. When I got to my destination, I opened the inspection plate on the diesel tank and pulled out handfuls of sawdust and plastic shavings.
Not sure why manufacturers don't do better quality control. These things don't do sales any good.