I’ve had similar problems where the thermistor said it was 30 degrees warmer than it actually was. I verified this with an infrared heat sensor. New thermistor solved my problem, probably was a faulty cable.
After you do all the easy checks…like making sure there are no stringy bits binding the filament in the bowden tube, and after checking and maybe cleaning the extruder gear, and trying a different nozzle…I would think that it is likely to be the teflon coupler that needs replacing. They are a consumable item and I have been through a couple.
I had a similar problem and it turned out to be the extruder. The gears inside the new feeder had begun to push each other apart sideways, down the shafts. As a result, the gear on the stepper motor had moved down the shaft toward the motor, resulting in a loss of movement to the filament. At first I thought it was grinding the filament, but there were no marks - it simply stopped turning the gear.
To fix it, I have applied glue to the shaft of the stepper motor and reattached the gear, in the same position down the shaft as in the upgrade video. As a precaution, I have also glued the bigger gear onto its shaft end.
Thanks to all of you! Finally, I solved the problem. I disassembled and checked all the hotend. The problem was some PLA wedged between the nozzle and the Teflon.