Wireless Help Needed

I've got a couple of questions regarding some wireless issues I've been seeing on some of the Macs I'm supporting. We're using a WPA enterprise encryption setup with Cisco Clean Access routers at the moment. With both 10.4 and 10.5 on occasion I've had at least three users, 1 with 10.4.11 and 2 with 10.5.X, have issues with what appears to be DHCP leasing. The users are either able to initially pull down our wireless certificate, authenticate to the wireless network, browse for maybe a minute or two, then they are unable to browse using the standard domain names and have to use Ip address of various sites. The other scenario is that the users can't authenticate to the wireless network even though they were previously able to and currently have a DHCP lease and an Ip address present. For both situations I've tried manually setting the configuration methods to bootp and then back to dhcp for the wireless card using the ipconfig command in terminal, I've done the same thing using the GUI interface in the network pane, I've done a safe boot and then immediate restart, I've used third party software to try and obtain a new dhcp lease, I've even gone as far as completely removing the LoginKeychain file from /Users/library/keychains. At least two of the users have airport networks at home and have reported that they don't have any issues using those networks. I'm new to supporting Macs and feel like I've hit the basics so any advice would be appreciated.