No MAC cloning on my network and no other DHCP servers that I'm aware of. Decided that I'm going to do some troubleshooting tonight when I get home from work. I'll pull the cable between the modem and router and try to isolate the problem. Maybe bring the whole network down and then back up one device at a time while watching IP addresses.
I'll check back in with the results.
Wireshark will be a big help here. Run it on one of your PCs and you'll see the DHCP replies as well as ARP requests from the DHCP server. You should be able to track down the offending device pretty quickly by the MAC address.
You can see that the motorola device (in my case a moto SB6580) is replying to the DHCP request from my laptop, as well as arping for addresses in the 192.168.0.0/24 network. Comparing the source MAC address(Ethernet II) for the DHCP reply, or the ARP request to the MAC address of the PC NIC or the cable modem/router MAC address should help you find the offender.