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Fast is stable then very unstable, the reason I think, is this growth of their indexing compared to their growth of hardware to store it. I've experienced it many times with fast. You will note that Google which is my favorite is more stable, because they keep expanding their server farms.
Quick way back to the top is some content changes in your top pages and resubmit them. Usually works if they were good enough to get their, before. This is a possible reason and answer - maybe some of the seniors members, moderators and admin chaps on the forum have better ideas.