Raft extrusion problem while printing with PLA


Hi, I have some problems while extruding just the first layers of the raft using an external PLA filament. The extrtuder seems like chattering with the material that can’t flow continuously. Obviously also the quality of the raft gets worse. As extrusion temp I used 210C.
This problem is not present while printing with Z-Ultra T. To solve it I tried to calibrate the table and to increase the Z-Offset but I didn’t obtain any results. The printer is pretty new with less of 100hr of printing. What could be the cause of such a problem?
I uploaded a video in google drive: https://drive.google.com/open?id=1CwRmeQYH30xvz5UJupL8v2nuBEcQB7Xr

Thank you in advance