ill try to explain this the best I can:
i bought a Taz6 around a month ago.
everything was perfect. until today, right after printing an object. The Z axis started to misbehave.
the extruder unit cannot go up or down. the z axis motors do not move smoothly. they vibrate a lot and turn very erratically.
and most of the time do not move.
I tried aligning the 2 Z axis sides manually by turning the motors after disabling the steppers, that helps for a couple of seconds and then it went back to not working.
i am not sure what else i need to do.
here are 2 videos of whats going on:
https://www.youtube.com/watch?v=mV8KGP0ZMa4
https://www.youtube.com/watch?v=CHrpczw-iSc
I tried this approach:
“Your TAZ 6 is slightly out of square, a known issue that sometimes happens during shipping.”
https://ohai.lulzbot.com/project/squaring-taz-6-frame/service-bulletins/
But it didn’t help.
Your x axis is either out of alignment and binding, or its an electrical issue. Repeat the step that measures the distance of the x ends to the measurement piece on the frame, ensure you are doing so with utmost precision (even 1/8" out can cause binding) then check the setscrews on the z motor couplers to ensure one isn’t slipping. Finally undo one side of the setscrews that hold the 12mm rods and attempt to run the carriage up and down again. When it gets to the middle of z, re tighten the screws.
If it still doesn’t work call support.
RESOLVED.
it turned out to be a problem with the RAMBo.
The Z microstep motor driver came with a heat sink on it, on my board this heat sink became loose and slipped off the microstep motor driver. This caused a short circuit as it was connecting with some other electric components.
Once the heat sink was put back in its original place, the Z axis problem disappeared.
I now have another problem, with the auto levelling routine that happens before every print. It seems that the “short circuit” cause some damage on the RAMBo - so it will have to be replaced 
I swapped the the old RAMBo with the new one that the guys at Lulzbot sent me.
Everything works perfectly now. I love the TAZ 
The support team at Lulzbot are extremely helpful. Thank you guys.
