This thread still getting posts! Wow.
Don't have any info on what causes it on this scope but I think a good starting point would be to read up on what causes SPC failure on other tek scopes, the more recent, the better, and look for similarities in the analog front end for your scope vs. what you can find on older scopes.
I have a couple of these scopes now, needing small work, none have some catastrophic failure (like no boot/blank screen/etc) even after being switched off for years; these don't seem to suffer memory retention issues like the low quality Agilent -X series scopes. I will want to resurrect my scopes someday, I really don't think a scope of this quality has been made since these. I have had Agilent/Keysight MSO-X 500MHz scope on my bench for some time and I am getting tired of this cost-reduced garbage.
I welcome anyone with further information about these scopes to post here, architectural information, knowledge of firmware, or any information regarding fixes to common problems.