Fixing the belts is a problem that needs to be dealt with, and it may fix belt slip, if that it what you have, but I suspect the real problem is motor slip due to low torque.
In the CoreXY mechanism there are fixed length belt segments and variable length belt segments. The fixed length segments run from the motors to the pulleys in opposite corners of the frame, and the segments that go between the fixed pulleys opposite the motors. No matter where the extruder carriage is/goes, those segments don't change in length because the motor are fixed and the pulleys are fixed. The fixed segments don't have to be parallel to the axes. The variable length segments connect to the extruder carriage and the pulleys on the ends of the X axis. Those segments vary in length depending on where the extruder carriage goes. The variable length segments must remain parallel to their respective axes at all positions of the Y and X axes. If the belts aren't parallel to their axes, the prints will be distorted, and belt tension will vary with the position of the extruder carriage (which could cause belt slip).
Ultra MegaMax Dominator 3D printer: [
drmrehorst.blogspot.com]