When that happened to me it was because the y axis belt was too loose. I had to install a tension device. It could also be from the motor slipping due to over heating, the pully slipping on the motor shaft as well.

Anyone purchase a chimera hot end recently? I was following the assembly instructions and there is a picture of them using a wrench to tighten the heat break to the heater block. Closer look showed two flat edges near the threads. My heat breaks have no flat edges. I would need to grip it with pliers which will cut up the part that slides into the cooler block. Any suggestions?