I get this loads. I have around 600 indexed pages now and some are always going missing then re-appearing at the next spider. No links change on the site..just happens with Google in my experience.
I include a unique “key” in the html to group my pages in batches (ie “xfg908d”, “dfg8889d” etc) that no-one else uses so i can group the pages and search for them regularly. I someimes find they are still there, just way down the lists, then they pop up again.