jimhar,
I will need to know a little bit more about your set up. Are you doing a finite or continuous pattern output generation? Are you sending 8 bit, 16 bit or 32 bit samples? Why do you say that you are limited to 2 MHz? Are you getting an error of some kind when you go above that frequency? Does your external clock look pretty good or does it have some noise?
Jeff Tipps
Applications Engineer
National Instruments