The sessions table only holds a finite number - this gradually shrinks as time passes but then increases again as people revisit the site, hence the sporadic nature of it. I've cleared the table now and will put something in to limit the chance of it maxing out.
I've just spotted the 'There are currently no topics in this forum - start one!' too, on the main page linking to TV Home Forum. For a moment, I thought all the data had been wiped!
Does this just happen to me or is it for everyone else that when I post in my own topic in The Gallery I have to put the thumbnail image URL in again otherwise it gets deleted?