Mate goes unresposive when ethernet network is disabled

Hello all! I have recently run into a networking issue and need assistance.

RPI 3 with Ubuntu Mate 16.04.xx plus all updates to current.

Both ethernet and wifi work when they are both enabled but ethernet is default for internet routing. My ethernet router just died so I switch the Pi over to wifi. The wifi is on, connected and working but the pi still thinks the ethernet interface is default and will not route properly. When I disable the ethernet interface, the pi chooses wifi as default and routing starts to work perfectly. However, the Pi is then locked in perpetual slow mode.

CPUs are running fine at about <20% utilization
Memory is riding smooth at about 25%
The mouse is free to move
Mouse clicks take minutes to respond
I can not find a rogue process
All ip addresses and routing are correct
As long as the ethernet interface is enabled, I get no network activity on the wifi interface even though it is on, reachable and configured correctly.
All things considered, the system does not appear to be hung except it is so slow to respond. If I can manage to re-enable the ethernet interface, the system immediately starts responding again but the internet will not route using the wifi interface.

Again, when wifi is on, ethernet is off, the internet routes and works perfectly but the Pi is almost completely disabled and unresponsive until I re-enable the ethernet interface. Once enabled, the pi is fine and fully functional except it will no longer route to the internet using the wifi interface.

Thanks for your assistance.