First of all sorry it's taken me so long to get back to you - been a bit busy !
The first time this happened i had 12 Mini Macs patched starting at 49 and the second was with 8 Mac 500's starting at 49. From memory i think there were dimmers patched 1to1 on 1 thru 48. I am going back a while now but it did seem very strange at the time. Sorry to be so vague but at the time i didn't have time to really look at the problem - i should be using the first desk again next week so i'll see if i can recreate the same thing ?