During the last days I did a lot of tests trying to solve the problem mentioned here.
If you're using the Mighty-1284p files by maniacbug, select the board "Mighty 1284p 16 MHz w/Optiboot" then put a 220K resistor in series on the line beetween the FT232 TX and the 1284p RX0 pins.
If you're using the "Original Mighty 1284p" board, then you could try with a 120K R. Or, maybe, without components.