Many people were having problems with reliable serial communications on their 1284P chips. Several hardware 'fixes' were tried with varying results. But what finally seems to really solve the problem was changing the low fuse byte value to use 'full swing' crystal option, this seemed to fix the problem for everyone that was having the problem.
So check what value of your low fuse byte is and if it presently has a value of 0xFF change it to 0xF7
Lefty