I have a pretty vanilla system. Windows 7 64-bit, no router, wired right into new modem, bridged modem, and just avast installed now (standard settings) and still having this issue.
I think Sonic really needs to look into this.
Windows directly to a router in bridge mode is super risky, from a virus and windows bug perspective.
Hide that machine behind a router... and as a bonus your DHCP problem will likely go away.
Sonic would be within rights to discourage such a setup (just based on the potential for virus / mallware if nothing else).