It is a known problem that the FrSky D4R-II has a bug with its PPM output mode. The PPM frame is too short at 18ms so when using all 8 channels it is possible to run out of room within the frame causing in my case crashes. FrSky have a solution with a firmware update that sets the frame to 27ms long.
Here is my video explaining it all in detail.
Here are the links to the articles on another explanation and how to update the firmware using an FTDI programmer rather than the special FrSky programmer.
You will also need to get the instructions and firmware for your reciever from the FrSky downloads page here.