Whatever else fails, I am glad to see that Karen's sense of humour never does.
The plan is to
-Load meaningful / recognisable data into the screen ram with the VDU disabled.
-Enable the VDU and see if the loaded content survives and is displayed in some form or is erased / scrambled.
-Take a bit of off-screen video of what's happening
-View it in VLC and run it in 'slow motion' to see if that makes anything more obvious.