It is possible for a motion controller to consume commands out of the
buffer faster than the host computer puts them into it. This is more
likely for slow computers and jobs that have lots of very short moves.
The solution in this case is to make the motion queue larger. The PMDX
plug-in defaults to a G-code motion buffer of length 250. You can edit
this using:
Config > Plug-ins > PMDX-SmartBOB and choose the Performance Settings
tab.
A larger buffer will help prevent under-run, but will also lengthen the
response time to Feed Hold and Stop commands from the console.