I got the same problem when I used a standard 6 conductor telephone cord. Once I discovered it wasn't wired the same on both ends (2 wires were crossed), I cut off and rewired one end so that it was just like the other end. It worked fine then. You should join the yahoo group for your radio. The address ishttp://groups.yahoo.com/group/FT-8800R
If you search the messages on the FT-8800R group, you will find allot of info about the seperation cable. I didn't buy the kit, but I made mine myself for less than $20. It took about 1 1/2 hrs. to do!
See it here:http://www.radioreference.com/forums/showthread.php?t=20217