That does appear to be the problem. I was basically sending that data as fast as my hardware would allow, I would poll my TX buffer and when it was empty, I'd drop the next value in. I first tried adding a pause equal to a few bit times but when that didn't work, I tried pausing for what would be equivalent to the entire length of a channel and then the laser was able to keep up.

Is this common in DMX fixtures? What is the worst case I should design for?

Thanks
Pete