ibeau - 2:24 am on May 8, 2012 (gmt 0)
I've noticed this over the past 3 weeks also, but in AUS. Long tail keywords are ranking well but highly competitive keywords are jumping all over the place and even disappearing. I don't think I'm being penalized but like tedster said, perhaps something to do with Google's learning algorithm.
I'm continuing to add fresh content and genuine backlinks so I guess I have to just wait and see..