Upgrade results in distorted colours and flickering when moving the mouse; functioned fine before upgrading to Xubuntu 18.
The Radeon x300 driver installed. Checked that vga cable is securely plugged in. Trigger to upgrade was a large overlay appearing of the Firefox address bar randomly over pages.
The PC is a dual boot old Dell XPS, no problem on windows side.
Another point: I triggered the login screen on reboot, graphics here were fine. Logged in and back to psychedelic colours and fuzzy text.