Two things
Which browser are you using? VBulletin isn't optimised for IE. IE being a giant piece of **** and all that. Also, may have issues with Google's Chrome as it's new.
Secondly, you should be autoclearing cookies in your history after a MAX of 7 days for high traffic sites like this one. (AKA, site you goto a lot)
It only takes a single character to be out of place in the cookie, which can happen for any number of reasons and the cookie will only partially work.
Wiping your cookies and checking your security settings will fix all your issues.
<I find myself wishing I had a dollar for everytime I explained this>