I have found a possible bug which displays wrong title in the tooltip. On mouse over, the title is not actually the title of the displayed thumb. Accordingly, all thumbs have the same title, which in turn, is the title of the current post/page!
(I use the php code manually on some pages.)
The site is a test site and it’s under maintenance at the moment. Therefore, not reachable if you’re not logged in. I’ll try to provide you screenshots if it helps for now or I should get back when it’s up an running.