Hi Charlie
Just a thought. My Rumba came with a spare SD shield. I assume yours did too? Try disabling your graphics controller in firmware and use the SD shield instead. You can initiate the SD print from RH or Pronterface. It would take that aspect out of the chain. The most likely cause is that the Arduino isn't getting data fast enough so the SD data path must be a good place to start diagnosis.
Andy
Just a thought. My Rumba came with a spare SD shield. I assume yours did too? Try disabling your graphics controller in firmware and use the SD shield instead. You can initiate the SD print from RH or Pronterface. It would take that aspect out of the chain. The most likely cause is that the Arduino isn't getting data fast enough so the SD data path must be a good place to start diagnosis.
Andy