No ads so this is definately a penguin issue
Very wise words, thanks
So I went for a walk and all those posts whirling round in my head seemed to fall into place.... so here goes a theory
Penguin is not just about backlinks its about a set of indicators triggered by spam websites. Imagine your site is scored for spam , after you reach a tipping point penguin is triggered and your site is demoted in the SERPS. For some sites this may indeed be spam links that cause the problem but other sites it may be onsite factors, in most cases it will be a combination of both.
Add to this a lack of strong signals that do not indicate the site is spam.
By looking at penguin like this it would be neccessary to look at the bigger picture concerning your site.
Take keyword spamming , its entirely possible this has been redifined and could now include keyword stemming. Overuse of a keyword that would be grouped in a venn diagram (thanks Leosghost) could cause an increase in your spam score.
IP address, beeing hosted on the same ip as known spam sites could see your spam score increased as well and a whole load more factors to do with your site could each individually push your spam score towards the tipping point that is penguin.
Anyway its a theory