I've got a CS 8000i that gets JAM 1314 when doing 2 sided using drawer 3&4. It will duplex fine as long as it's 5 or less but the moment you try for 6 or more it jams. Both drawers 1&2 work fine when duplexing. So far I've replaced the middle roller unit and the exit assy after the fuser. The machine did just have the fuser replaced due to it hitting the maintenance count. Any help here would be great, thanks.

Other machines in this situation would mean that the is a oneway or clutch slipping. The timing has to be perfect for two sided copies.