I use an ad service called Broadstreet, and they have a plugin that gives me more direct control of the ads on my site. That plugin seems to be conflicting with my site, but not all the time, only certain posts.
Oh, sorry. I actually deactivated the plugin again while I was talking to Broadstreet, and then I got called away and didn’t reactivate it. So that’s why you weren’t seeing the issue. I turned it back on for now. Broadstreet has forwarded the info to their dev team, so we’ll see what they can deduce.