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In my view, it's not a big problem to have these sites on the same server, as long as each site has unique content, and there isn't a huge amount of inter-linking between the sites. (in which case, 30 sites would sound a lot like a link network to me, which would risk all of your sites being de-indexed) What is potentially a problem is making sure each site is truly unique. This means having unique content (including navigation and in some cases, unique templates - although some would argue this is a bit extreme) You say that the sites employ the same link building methods - does this mean they all have links from the exact same sources? Again, you could be stumbling into link network territory again... or Google may choose to place less value on those links. It's a murky area, and you'll have to use your own analytics and judgement to tease the answers out of Google. I'd say your not in trouble just because of the shared IP, but my suspicion tells me your in a dangerous neighborhood. Hope this helps. Best of luck with your SEO!

I saw ranking changed every week for the things. Google is very frequent as far as updations are concerned. Their are few updates as well like Panda update which it does periodically like in half year.