There is definitely a universal issue about transmitters and receivers not matching in rare cases. I suggest you increase BREAK to 100mics and MAB to 16mics to 20mics.

There may be also a classic fault of 250KHz NOT being quite 250KHz but a little more..... this would send in the first few channels OK after SC and then , gather error as it moves down the line.

Every processor has it's own little quirky way of doing the 250KHz best in only one possible way.

Forexample, in a 89C51 you can do it best with a 16MHz Xtal and using serial port set to Mode 2 with PCON = h00. This will set Baud rate = CLock Frequency / 64 = 250KHz very precisely for DMX transmission. NO other configuration seems to give the right timing

BTW, check your XTAL anyway. They sometimes screw up very innocently.

Ujjal