Since posting my reply, I had the opportunity to talk with the US sales rep.

Apparently, with the OpenDMX, the computer is having to do all the timing chores in software. How close the result is to the "standard" 44 frames per second depends on how much other stuff the computer is trying to do at the same time. When too many demands are made on the computer, he said it starts "dropping" packets.

He also suggested that the USB DMX Pro is a way to eliminate the problem, for $95 US over the cost of the OpenDMX.

http://www.enttec.com/index.php?main...name=dmxusbpro

Thanks for helping me out with this!