Y axis cuts inconsistant, progressively worse over course of job. Chinese Trocen

Hello all,
I am having issues with y axis inconsistency that gets worse and worse over the course of a job.
for example, in the photos attached I cut a set of dials. the center holes were cut after engraving and became more and more off center as the cutting head moved back up the piece from the bottom. then when the dials were cut out, the circles being cut became more and more out of sync until there is an obvious gap where the end point should meet the start. have already tried realignment of both laser beam and gantry.. is this a stepper motor or controller problem?
thank you for any help!

Chinese 50w Trocen controlled 500x300mm.
maybe I have messed up the settings somehow? engraving and general line tracing seems to work fine...