steve8383 - 5:51 am on Jan 26, 2012 (gmt 0)
Fairly sure it can't be my affiliate offers. It's a travel based site so I offer lots of hotels and tours. Typical page is written description details mixed with photos, maybe an embedded video with a simple text link to the booking page of the site I am affiliated with. The (affiliate) text link is more often than not well below the fold.
I've currently disabled my leaderboard adsense on the forum pages. Also realised that I have another 728*90 between first and second post (also disabled) plus a third as a footer. So, when a thread is new and has no replies there may be 3 ads within the fold. When a thread has several replies (as most do) it doesn't look so ad heavy.
Hopefully, my rankings will come back now I've disabled the ads. I'll report back.