I am having a simular problem on a 4507R. The only exceptoin is that my clients are also going through Cisco 7960 VoIP phones. The client works fine, then, at random, will no longer communicate via network. IP address and DNS are still valid on the system. If I renew the IP address, or repair the network connectoin, everything works fine for a while.
I also have forced the client and the phone to 100Full to try to fix the issue.
Anyone have any suggestions?