klark0 - 3:19 am on Feb 2, 2012 (gmt 0)
Definitely a penalty. I had something exactly like what you describe a while ago.
The only fix was several reconsideration requests.
I had many redirects to affiliate offers on the pages, so I first thought that was the cause. I fixed that then sent the 1st request. No luck. Then I sent another and another and then another, each time detailing different changes I've made based on my guesstimates of what might have caused the penalty.
And then finally on the fifth try, I suggested that a disgruntled potential buyer may have done "things" deliberately to harm the site after i refused to sell it for what he was offering.
That worked! The 5th request was approved and the site returned to it's exact rankings before the penalty and then started moving up the following week like nothing happened. It's now #1 for over 40 keywords with 10,000 visits per day from Google alone. Needless to say I'm glad I didn't sell.
And yes I did build some questionable links which is probably what led to the penalty. But I really did a have a pissed off buyer who thought I was asking for too much. So technically, I didn't lie to google :P