Make sure that you have the FC8600 selected in Flexi and not the FC8600 GPGL.
Also, this could be caused by noise on your data cable. If you're using USB, make sure that you don't have any extensions or hubs between the plotter and the computer. Try a new USB cable. Check for updated USB drivers at GraphtecAmerica.com.
If you're connected via Ethernet cable, try connecting directly from the computer to the plotter, not going through any network or switches. Try a new Cat6 cable - better noise rejection than Cat 5 or lower.
Good Luck