results disappear from spotlight searches

I have a large number of pdfs and use my laptop as a personal search engine for work. In the past year, a problem keeps happening with Spotlight. Searches generate a list of files that contain the given word/phrase (usually correctly) but these then rapidly disappear one by one after a few seconds. When I repeat the search, they reappear -- sometimes fleetingly, but usually I can eventually get the correct list to stabilize.

What's going on? Anyone had the same issue?

I've done to re-index trick of the whole hardrive in the Spotlight several times.