Check for Q458.
I've repaired a T935 some weeks ago. Symptoms: -2000 V sometimes disappeared.
Turning the scope off, waiting a few seconds, then turning on again, sometimes it worked sometimes not. Then he died completely.
Pulled out Q458: was almost short.Measuring the resistance between C and E showed a few hundred ohms.
Replaced with a BD410 I had on hand, the scope started working properly again. Original transistor was TIP50. It was badly fixed on the heathsink so it probably got hot and stressed over time.
P.S. i'm from Italy.