The first I would look at switch logs an check spanning tree to see if loopsAlso needed to look at the topology of the 20 switchesSecond would be check interfaces of the server and clients for errors and loadSent from Cisco Technical Support iPad App

If you set a static address on a pc and attached to the switch can it ping the dhcp serverCan the switch management address ping the dhcp server and are they in the same subnetSent from Cisco Technical Support iPad App

It could have some thing to do with the iOS or device typeCan you not get to the dmz from the Internet not using the VPN ?Sent from Cisco Technical Support iPad App

Could you not have a second 2600 and split the load between them amnd use routing protocol between themSent from Cisco Technical Support iPad App

What ip address are you using on the asa ->edge router ?How would this addressing be used with your second ISP ?Just some things to think aboutSent from Cisco Technical Support iPad App