At 490Hz the 0.5ms time to print shouldn't matter though. Plenty of time to do it and move on, right? I also tried a little less than 400Hz and had the same problem. And actually, I noticed the problem without the serial commands. The example code I posted is just the easiest way to demonstrate the problem. The program I wrote worked fine on the UNO, but not on the DUE. The only reason I need the DUE is that I need more memory than the UNO has available.