The first thing you should do is disable to plug-in system to see if it's a modification doing this.
http://www.vbulletin.com/forum/showt...plug-in+system
This won't require you having to re-install everything, this is just a quick check to see if there is a real problem or just something you've added. If doing this does solve the problem then you should turn plug-in's back on and enter the plug-in manager. Disable plug-ins one by one until you find the one that is giving you trouble.
If turning off plug-ins doesn't solve the issue, then you have a problem elsewhere. Also, have you installed any source code modifications?