Y-Axis Drift

I am a new LulzBot Taz4 user. I am consistently running into an issue when printing parts taller than a .25 inch. The Y-Axis (Moving build plate) seems to drift, but only in this direction. See pic below. Using Sclic3r V1.0, ABS Fine (no support) configs from Lulzbot website. Ideas?

On further investigation, it appears the y-axis pulley on the motor may have been loose. I tightened it further and running a test print.

That can do it- as can loose belts. If you encounter that again, check the belt tension next, or belt path to make sure it’s not riding too high/low and rubbing against something.