The EHT is arcing somewhere, open it up and look for obvious signs of carbonisation, loose EHT connector etc.
Turning it on in a darkened room with the covers off may enable you to see where the problem is.
However, be careful - the anode voltage in a 'scope is anything up to 20kV, don't probe, prod or touch if you are unsure what you are doing, and remember that high voltages might be present even with the power turned off.