oric_dan:
Hi leo, I don't understand. You're actually bridging the Rx and Tx pins using a 220K resistor?
No. I put a 175K in series on the line between the Arduino pin and the 1284 pin.
Arduino RX ------ R ------- RX0 1284P
What does that do? Where did that idea come from?
A friend of mine investigated the problem using an oscilloscope and he went to the result that the 1284p bootloader based on the Optiboot 4.5 works too early. It sends the reset and then start sending the datas on the line. A R introduces a little retard on the transmission, helping the bootloader to properly receive the datas.
Also, isn't 175K greater than the 1284P program space?
I was talking about R values, not firmware sizes 
Also, I don't understand why the UNO board is in there during sketch uploading. ??
I don't have an USB/serial board, so I removed the chip from my Arduino and I use the Atmega8U2 as a USB/serial converter.
Also, I think the 1284P optiboot uses 115,200.
The Optiboot 4.5 works at 115,200, the other bootloader included in the maniacbug's core is based on the Stk500v2 bootloader from Atmel, also used on oldest Arduinos (like 2009). It works at 57,600 and has a better management of the timings.
So, I don't understand anything here. [early morning] ?????
I hope I have explained better 