If your job has lots of short moves and your computer is on the slow side,
then it is possible to consume the motion queue faster than the host can
build it at times. There may be other faults that can cause this error but
this is a good first guess.
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
Config > Plug-ins > PMDX-SmartBOB and choose the Performance Settings
A larger buffer will help prevent under-run, but will also lengthen the
response time to Feed Hold and Stop commands from the console.