Yes, its frustrating as hell, I simply don't have the knowledge to pin point a cause hence the post on here hoping the guru's on here could make suggestions.
Being that its graphic corruption I can only think of 3 thing, memory, Lisa, chip ram.
To make matters more awkward the machine isn't connected to the internet so getting programs on to the machine is difficult / near impossible as I have no other hardware to do this. This stops me testing the ram unless there's a ram tester built in to workbench 3.1 (its so long since I've used the machine).
I'll post a phone pic of the problem I have, its just weird that it manifests itself as jibberish boxes that go away which make me think of a ram issue.
Lastly, I hate smd work