[ubuntu_studio] 14.04.4 live usb session: randomly missing characters

When the login dialog or the desktop appears, menu and icon text characters are randomly missing. Any ideas why, please?

The first time I tried ruing the live distro, I was presented with a confusing login dialog box with characters missing, so I rebooted in despair.

Then I tried vga=771 as a boot option for troublesome displays, but it made no difference, although the login box didn't appear that time.

Basically, something's causing text to be rendered incorrectly. It's an unmodified Dell XPS8700. I am trying to escape a failed Windows 10 auto update, hope I don't have to go back to it!

Kind regards,
Matt.