Problem with IP address for WiFi and Ethernet - USB Conflict?

Hi,

I have started getting problems with my wifi connection on my late 2011 iMac running Yosemite. Every so often the connection will stop and when I go into the Network Preferences it says that there is no IP address for wifi and also for my ethernet connection. I tried removing and reinstalling these services, and also renewing the DHCP to no avail. Then I eventually discovered that when I disconnect my powered USB hub (used to connect several external HDs) from one of the iMac's USB ports the wifi immediately reconnects. I can then plug it back in and all seems fine ... until next time.

If anyone can help explain what might be going wrong and how I can fix it more permanently I'd be very grateful.

Thanks,

Robert