I have a VPN5002 and a IOS router (ubr904) with IPSec support connected by an IPSec Tunnel. And I have the same problem. These IPSec tunnel goes down after (approx) 2 hours connected. So I must reboot the VPN Server to my tunnel comes up. I saw that my IOS Router has an IPSec lifetime, but it is not the reason for the problem. I have no connection problems with VPN Clients and the VPN5002 Concentrator.
Thanks.