nobody knows if your site are stable, as what i know those authority sites are sites that has stable rank. if you want your site be stable on top, just continue to link build your site by getting quality backlinks and creation of unique content.
It really depends on how good the content of your site really is. If search engines like Google and Yahoo think it should be on page 1 for the keyword, then it will definitely stay there
How secure do you feel your rankings are?
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