ckissi - 6:19 pm on Feb 27, 2011 (gmt 0)
@valex, I know what I'm talking about because I got such penalty 2 times 2 years ago. Exactly the same, some keywords weren't touched and some of them moved few pages down (from my top 10 about 50% moved down).
It took about 2 weeks then recovered. Actually traffic recovered to about 95% of what it was before.
I didn't got yo-yo effect (thanx god ;)) but the friend of mine is experiencing same thing. He tried to make tons of different modification on site itself but he was never able to get rid of this.
So I assume its something related to inbound links or something off site related.