I recently upgraded from an old PRT (serial port old) to the new RBK control system. I have noticed that machine does not cut as smoothly as it use to cut.
I had to adjust the drivers for the motors and set them according to per the recommendations from the Tech Team at ShopBot.
I am noticing what I am calling a "Chatter" for lack of a better word. Also the machine will move slow the quickly then slowly again.
Here is a link to a video (youtube) of how the machine is moving. Also attached are some pictures of the "chatter".
Did you load the proper machine settings file and firmware for what you have now, a RBK system? Your slow and go motion is likely from wrong ramp values. The right settings file will set those to the proper default. Also, verify that your machine doesn't have slop in all axes and is not binding along it's travel, pinion grub screws tight on the motors, and motors/pinions engaged into the rack with proper tension. Then report back.