You sure you've got your iMac set up to get a dynamic IP address from the router's DHCP server? Sounds like you might be getting a clash of IPs on your network? Difficult to tell without seeing the iMac and PC. Maybe when I'm down in April I'll have a look if it's still a problem?
Paul Clarke's blog - I live in Hythe, Kent. Wed to Clare + father to 2, I am a full-stack web engineer, and I do js / nodejs, some ruby, other languages ect ect. I like pubs, running, eating, home-automation and other diy stuff, history, tree stuff, TV, squirrels, pirates, lego, + TIME TRAVEL.