User logout issue

Is it per chance a web based application that supports cookies? It could very well be that source address persistence is failing you. Internal users would normally have static addresses that don't change over the life of an application session. But an external Internet user could be going through all sorts of devices along its path that could be changing its source address. Such a thing would be intermittent, but would almost certainly happen. If the application is browser-based, cookie persistence is not only rock solid, but actually removes the burden of maintaining persistence table entries on the BIG-IP. Otherwise there are other very robust persistence mechanisms worth considering.