Network switching issues with MacOS 10.7?
I have seen something similar to this; I do not have a fix (wish I did).

The problem is that Lion speculatively brings up the interface immediately with its last known address (presumably based on the router or AP; it does store that information in one of the system plists), while querying the DHCP server to find out if that address is actually available.  If the DHCP server returns DHCPNAK, the interface becomes unconfigured while it properly negotiates a new address, at which point the interface is brought back up.

Theoretically (although probably not in practice on a small home network) your computer could have the same address as another computer on the network during that initial speculative phase.