I have just recently installed 3 managed Cisco SG500 series stacked switches in my environment. I have a Windows 2008 R2 DHCP server, which all user's machines use for addressing (Windows XP and Windows 7 machines). The DHCP scope is only about 75% utilized, but ever since these switches went in, I have had random machines intermittently lose connection. When this happens I have had my users try rebooting, which did nothing. Then, at the machines myself, I have attempted ipconfig /release and /renew, also to no avail. If I set a static address, then everything works fine. On some of the machines, I have just left DHCP configured and walked away to work on something else, only to come back 10 or 15 minutes later with everything working fine. What could be causing this? Currently, all switch ports are set as trunks with the Administrative PVID set to the default VLAN (these are default settings from Cisco). I will look back at my DHCP server again, but I am lost on what could be causing this issue. Can someone please help?