Has anyone had any problems with IE8 and sessions? In particular Sharepoint or OWA sessions?

We have a couple of county bods who work at the school and they access their county email and extranet from our site. Previously all was fine and then not long after we upgraded to IE8 they came and said they can't use their systems as it asks them to re-logon at random times. This can be just after they have logged on, if they click the new mail button, or go to another area of the site. By going back and trying again a few times it sometimes lets them through. Using our on OWA systems works fine.

To me it looks like a sessions issue but County have no advice and either pass me round their departments or say it is a client side issue so I have to deal with it.

I have added their sites to the trusted list which worked for a week, checked different configurations of GPOs and can't find anything that solves it.

Having a search says it could be ISA causing issues with sessions but we don't have ISA here it is at County.

Any ideas for fixes or what it could be?