Print jumps 7mm in Y direction after initial layers

[printbus]

... I do not have any active cooling on any of the stepper motors themselves.

...I think the belt is heating up and becoming less rigid and allowing a little more flexibility in the belt.

One thought is to check how hot the Y-motor gets during your print. For those of us with the "hot" running motors, I found the Y-motor got the hottest of them all, likely since it is so boxed in when mounted in the frame. Maybe that heat is causing the Y-motor to skip or is soaking through the pulley to the belt?

For a while I printed with a small desk fan angled towards the Y-motor, and then with a 40mm fan sitting on the table in front of the motor. Now the 40mm fan is mounted to a piece of wood that is in turn bolted to the bottom side of the v-rails. It only takes a small amount of airflow to keep the motor case at what feels like a reasonable temperature.

EDIT: More info on mounting a Y-motor cooling fan to the v-rails is in this post of my build thread - Y MOTOR COOLING FAN

[gmay3]

That's a good idea printbus! I will try moving the RAMPS fan to the Y motor to see if that helps.