In a previous life for many years I had a consulting company where I dealt with client offices, including system they set up themselves. What Jim told you, an IP collision, sounds 100% right, it would behave exactly as your describing and is one of those scenarios (two devices fighting for access and preempting each other) where there isn't a list of possible reasons, there is basically one, Jim's:
- Check your DHCP settings on your router, including any exclusion list (Most likely)
- Make sure if either of the devices are trying to use a static IP that in in the DHCP range (doubtful)
- Make sure you don't have 2 DHCP devices running (such as a cable modem and a router) that are offering the same address range (very unlikely)