spcomputing:
If you bootload/sketch upload this one on the 1284p UART1 (as opposed to UART0) it seems to work well.
Google Code Archive - Long-term storage for Google Code Project Hosting.
Just replace the "U1" optiboot in the folder of the Mighty1284 Optiboot and burn bootloader.
I had exactly the same problem: 1284p with FTDI serial converter. The bootloader was written correctly,
"blink" could be flashed but any larger sketch failed with a "protocol error" at various points of the upload
process ranging from 5% to 60%. (Previously I used the "Original Mighty 8 MHz" bootloader which was slow but usable. Ken Shirriff's IRremote- ibrary, however, insists on 16 MHz so I had to change to that bootloader version.)
I spend hours and fouled around with every parameter I could find: cabling, crystal, capacitors, voltage, software - nothing helped.
The UART1 bootloader solved the problem at the very first attempt...
Great - thanks!
Make it available to the general public!