So just to update this, I have done a fresh install of the 7.2.0 ISO onto two different USB sticks, just to rule out possible USB stick issues. Both installations have failed after a few minutes - the screen images deteriorate and the OS becomes unusable. I am now back to using a fresh install of 6.7.1 on one of these same USB sticks and it's been working fine for the last hour or so. Below are a couple of photos of what happens to the screen:

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