Bugger. Yes, these old computers can be touchy.
One of my System 80s "occasionally" will give a garbage screen for no known reason. Switching it off and on again usually clears it, but sometimes I need to do that several times. Then it will go for hours and hours with no problem at all. Then once in a blue moon throws its toys out of the cot.
Being so intermittent, Who knows what's wrong. Perhaps a bit of oxide in a socket somewhere. Perhaps a cap on the edge of it's tolerance level. Luckily 99.9% of the time it's fine. It's not the kind of fault you want appearing during disk writing operations though