there are a few distinct possible cases...  If the underruns occur in the middle of a voice transmission, you'd also expect to hear a "pop" in the audio.  This could suggest insufficient CPU.  If they occur only at the end of transmissions I think that's "normal" in the sense that the audio subsystem expects to receive a continuous stream of samples and gives a little notification whenever they stop.   It would also be interesting to know how close the system is to being CPU-limited.  Likewise, it would be good to know more about the received signal quality - occasional loss of signal lock could be another suspect...

Max