a lot of the weird issues that I have seen over the years in various models of Tektronix scopes from 2000 up to 2015 or so that had no other explanation
have ended up being due to RAM chips (or bad soldering of BGA chips to a lesser extent)
from all the description given, I can say with 80-90% confidence it's a ram issue. one if the SDRAM chips is going haywire.
I have seen cases that only a certain small range of addresses in a SDRAM IC had problem! very hard to find.
and no, even if you had the schematics it wouldn't help you that much. Certainly it wouldn't be a 10 min job at all. Believe me
perhaps that might have been true for old equipment with tons of transistor circuits and analog ICs and much simpler logic ICs...
