Question Y axis - one side goes the wrong way a bit before going correct way

This machine had been working fine for 6 months since I bought it ... but now it's giving me the poohs!!

I feel like I have been through everything and have adjusted, corrected again and again. I've taken the lead screws out about 10 times to decuple check that the alignment is correct. I've got it to the point where it actually moves but it seems that one side sometimes moves the wrong way just for a bit before it then goes the correct direction - obviously putting the gantry out of alignment again. See the video for a demo if the issue.

After many hours of trouble shooting it turns out that the stepper motor was faulty. I swapped the motors so that they were on opposite sides and found that the problem transferred to the other side. Stepcraft Australia sent me a replacement motor and bingo, it's running perfectly again!