Client Access Sessions Losing Connection To AS400

I am not very knowledgable about AS400s, but we are running Client Access V5R2 and are connecting to an AS400 running 5.3. We have been having a problem with users losing their sessions intermittently. The users are connected to the AS400 via Ethernet thru a switch and are running Windows XP, SP1. Generally, they will have 2 sessions running at a time, logged in as a different user (i.e., User1 running Program1 and User2 running Program2). Sometimes a user will lose both sessions, sometimes one will be lost while the other session stays up. There is a Cisco 2940 switch that sits between the clients and the AS400 and I do not see any loss of packets are any other indications of errors in the switch log. When the session goes down, any attempt by the user to reconnect displays a message that the "Device is invalid". As I said, this happens intermittently, but it does cause problems. I do not administer the AS400, only the Windows Network and the AS400 admin feels that it is either a network issue or a power issue. All users have surge protection and battery backup. I would think that any network issues would cause all sessions to go down and not just random sessions. Please let me know if there is any other information that I can provide and I will do my best to get the information. Thank you.