I agree with your analysis. So there are two possible reasons
3) Isolator - this is most common if you've printed extra hot (>240C) for a few hours or regular temps (220C) for 500 hours.
7) clogged nozzle -
I have not done printing at high temp so clogging is most likely the issue. Since I have tried cooling and pulling out filament, is there any video showing how to disassemble the extruder and do a more though cleaning? BTW, what official UM parts do you think I need to fix the issue ?