I just noticed that there's an instant preview "test'in the Webmaster Tools lab section. It shows three cases - on-demand, pre-render, and mobile.
I remember that when Google first introduced instant preview, it showed a kind of "Skewed" off-centered view of one of my site's pages, and I had to add a small top-centered image to every page to get it to come out right. But I don't remember the exact details.
Anyway, you might check out Webmaster Tools as part of your investigation.