Hi Bill,

Mach3 is known to have some CV (Constant Velocity) issues.  Have you tried adjusting the Mach3 CV Settings?  See this Thread:

It isn't likely to be an issue with data rate.  The example SimpleFormsCS.exe has a button to test your USB speed on your computer.  You can run it from the \KMotion\Release directory.  On my i7 system I get:

PC->KFLOP N=100000 Int32, Time=4.400 sec, 202KBytes/sec

HTH

Regards

TK