I have a problem in one of my sites since I changed the title for around 20.000 pages. I did it once and not a big change, just deleting one keyword, but it seems to have triggered some kind of filtering: these pages are gone from Google index, even searching for their unique piece of text.
Some of them come back to index for a short period of time and then disappear again. I have tried to change back the title for 3-4 of these pages and it worked, but only for some of them.
Any experience with this kind of problem?