Paradog is correct... there are several reasons for underrun, which is a condition in which data does not get to the CD recorder as fast as it needs it.

Suggestions:
- Plenty of RAM is good
- a fast processor is good
- do not run other programs at the same time as they use up available resources making it more likely for underruns to occur....
- turn off virus protection
- reduce burn speed

In general give the CDR as much of your machines resources as possible... newer CDR drives compensate to some degree by building in to them buffers in which data is retrieved and stored in the drive before it is needed... the rule of thumb is the faster the record speed the more resources it will need to be successful.