Continuous Generation in C - Speed issues

I'm working with NI PCI-6561 using C language (VS2010). My task is to stream data continuously from memory (12 channels, ~16MHz). I've modified "ContinuousGeneration-StreamFromMemory" example for my needs. Everything works perfectly on my dev PC (SandyBridge, i7), but I have problems with my program to run on industrial PC (axiomtek fab101 PCI backplane, Ivy Bridge, i5). It works only if I reduce clock speed to 4Mhz. Even at 8Mhz I have buffer underrun errors. Any ideas how I can improve my program? NI docs tells about DMA mode, but I can't find an example how to use it using C.