I feel your loneliness. diy forums all over the place seem to be dying . Unfortunately, I don't have the knowledge required to help you with your problem.

Although I doubt it's the stepper motor, and if you think it is, the obvious thing is to switch it with the other motor and see what happens. Then try swapping the drivers to see what happens. What would the filament cost be to run those tests? 3 to 4 hours into it, wow. Such a random thing.

I don't own a filament printer, so like I said, I don't have this specific knowledge to help you. What does a new print head cost?