post retire randomly

Ever since I’ve upgraded to v 3.5.1 random posts seem to randomly retire. There is no obvious pattern that I can identify as sometimes it’s just a couple of them other times loads more, sometimes at night or during the day, and it will affect older entries as well as more recent ones.

We’re using WP multisite and just one of the sub sites is experiencing this – the primary domain, if that matters at all.
In the DB the post_status is changed to ‘fwpretired’ and thus they disappear from the front end.

This is not good at all, especially for our business, as customers get newsletters and alerts linking to them, not to mention that the site can be penalized for the missing content.

Has anyone else experienced anything similar? would there be a server related issue or a plugin behaving badly after the update? This never happened before upgrading so there might be a direct link to this.

Any help is greatly appreciated.
Cheers.