Sounds like Z axis flex or X axis slack in the belt.
I found that accuracy improves a lot when you lift the cutting area by putting some MDF on top of your wasteboard. When the lower Z wheels align with the lower X gantry wheels I get the best results.
Have you checked the power consumption of your stepper motors? Uneven load on the Y motors can make them turn in the right direction but movement may be uneven.
If you doubt 1.1f you can go back to 1.0C to see if that supports your conlusion.