I get this from time to time if there are air bubbles in the lines. I am assuming you have the accordion style threaded grease tubes? If so look about an inch down from where it threads in for a grease zerk. Press the wrench key on the control, look for the aux menu and turn the grease pump on. While the grease pump is running ( you'll hear it) back of the grease zerk with a 5/16" wrench and bleed the air out of the system. You could also have a busted line, I had this on a 510c mill. A single chip made its way under the covers and up against one of the rubber (?) hydraulic lines for the grease and made just a large enough hole for it to detect a pressure drop (occasionally). Maybe pull back you way covers and look for a big glob of grease.
