iPursa Aluminium Pro z axis won't go up

[palma]

Originally posted as a reply to a similar topic but realized it should be a new post...

Recently purchased Geeetech Aluminium i3 Pursa pro
Great instructions and manual. It took only 6 hours to build and start the first test print. Everything went well other than needing to lubricate the lead screw and loosen the Z nut mounting screws. Made three successful prints of varying heights before it stopped printing anything over 5mm above the bed.
Problem = When trying to move the z axis up, it only goes 4-5mm before the left z motor starts vibrating while the right motor continues for a few more millimetres before the carriage jams.
Read all the forums and tried several things…

Inspected threads with a magnifying glass…No irregularities.
Disassembled the z lead screws….everything moves smoothly including the rods.
Swapped out the z axis plugs…no change.
Installed spring loaded z nut mounting screws …no change.
Loosened the x axis wing nut….no change.
Swapped out the left stepper motor with one I know is working….this worked initially, I was able to move the z axis all the way up and down, but after cycling the power supply and homing the printer, I was back to having the same problem. (This may be the key?)
Considering upgrading to anti backlash nuts and upper ball bearing supports for z screw. Does it need more powerful motors?
Any ideas?