I have a mixed environment of HP Procurve switches and Cisco Switches. I had no problem with DHCP conflicts up until recently when one of my procurves died and I replaced with a Cisco 3750X. Ever since then I have been having DHCP conflicts on end user machines quite frequently. When I check the DHCP server it shows an odd unique ID which is a bad MAC address that isn't even complete something like below

10.5.71.94   BAD_ADDRESS   3/10/2014  DHCP   5e47050a     This Address is already in use

I get several of these a day when I check my DHCP server. Nothing special about my switch. I do have a second VLAN, but it is a 192 network that is for my guest wireless. Before this switch I had only two other Cisco devices a 2960 and one other 3750. I do have spanning tree portfast turned on all ports with BPDU guard. The ports that are connected to a switch I have portfast turned off. The only thing I could think of was ARP. So I modified ARP timout all the way down to 0 and still got conflicts, but just not as often.

Has anyone seen this or have any insight as to what I can try? Thanks in advance.