First the original cracked (as I’m told is normal, so I replaced it with an aluminum one, and the spring are is flexing all over the place as it runs🙁
I watched the video. I see the wobble? The flex if that is a better term. If there was a differs angle maybe a better guess might be had.
The guess I would make is too much tension?
I also wonder is the bearing (I assume there is on in the arm) is not fitting the shaft it rides on. Or it does I can’t see if the movement is the idler or at the arm or both.
The video I watched said ‘tight, but not too tight’, this video is after I tightened it more, it was really flexing before. Maybe it’s because it’s a cheap amazon product?
Also, what I ‘think’ might have happened to the plastic extruder; a roll of filament I was running was looped under itself, the filament didn’t break and it ran like that for well over an hour.
The shaft of the motor could not possibly be bent, right?
Will probably pop into the store tomorrow and buy a new extruder and maybe motor 
Here is another video, hard to get in there and see what I’m filming…