Hi, My guess would be you have 2 default routes when both connections are enabled. To check this you should open a terminal .e.g gnome-terminal and ran sudo route -n and check if you have 2 default routes e.g.
0.0.0.0 192.168.1.254 0.0.0.0 UG 0 0 0 wlan0
0.0.0.0 192.168.1.254 0.0.0.0 UG 0 0 0 eth0
If so you need to remove the default route from the wired interface e.g eth0 if your using a static ip. If your using dhcp on the wired interface I suggest you setup a static ip via the /etc/network/interfaces file (example below)
iface eth0 inet static
You'll have to restart the network service after adding modifying the network file
sudo /etc/init.d/networking restart
Hope this helps.