I'm thinking this maybe more of a timing issue. I guess. What Arduino do you use? I made an Open DMX board using Arduino, first I use the old Diecimila board with ATMEGA 168 and it has some troubles, blinking erraticly to be exact. Then I switch to Duemilanove with ATMEGA 328 and the problem disappeared. From what I read, the ATMEGA 328-based Arduino board seems like the best option for DMX, not sure about the newer Arduinos