Random Logout of Users

Hi,

We have a medium/large WordPress Multisite installation. We've been having a problem lately, with our users getting randomly logged out while using the service. This can happen when they are composing posts or pages and modifying settings in the admin area. It's not often and it's not consistent, which is frustrating.

We're running WordPress 3.4.2 and have 575 sites and 2,743 users.

Any ideas on what we should be checking? Hardware/memory issue? Plugins or themes?

Has anyone else ever run into something similar.

Any advice is appreciated.

Sylvia