Try creating a default style and see if it still happens...
Create a new style with no parent - Styles & Templates > Style Manager > Add New Style > no parent - then browse the site using that totally default vbulletin style - do you still have the same problem?
Then this is a style problem and you will need to go through all your modified templates (they will show up as red in the templates list) and see what the differences are between your templates and the default Jelsoft one.