getting logged out

A few good hand of members have reported getting kicked out of my forum after a some time, while they're even writing up a post and about to submit. They end up losing what they've written. How can I trace this? Its sporadic to some (never happened to me before, using Firefox 2). They say they have cookies enabled too.

To be honest, I am posting here in case it might be vbseo causing this. I'm not sure.

My vBulletin cookie settings:

cookie session = 3600
path to save cookies = /
cookie domain = blank
gzip html output = no
gzip compression level = 0
add standard http headers = no
add no-cache http headers = yes
remove redirection message pages = yes

Try upgrading vBSEO to Rc8