Weird - Google Rankings Disappear/Reappear Trick

[apexis]

Has anyone else experienced this?
I have now seen 3 of my sites with very strong rankings in Google (top 5 for many competitive keywords) disappear completely one day and then immediately reappear the following day right back to where they belong. Each site experienced this once.
I don't sell links on these sites, they are not blogs, I don't do anything black hat.
Strange things going on with Google these days...