Hi Mel and welcome to the forum.
Nice write up of your CDU150 make me want to drag mine out of hiding
I am wondering if there isn't at least some resistance in the multiplier at the transformer end it might give the EHT generator circuit a bit of a hard time?
Those Fairchild U17718 are they matched pairs in a single can?
When I was at Plessey Avionics in the Mid/late 70's they were a thing, I guess improved manufacturing means its easier to match transistors although I have somewhere got a stack of multi transistors in SM packages.
Looking forward to hearing more about your Scope, then whats next I wonder
Cheers
Mike T