Test with MP3 and see if the problem happens with that as well. Any errors in the perl logs? Any other shared details between these calls aside from being incomplete?
Server load when these calls occurred and when the mix script runs? Note that this is not something stored in a log, you'll have to create one and store the value in a log somewhere so you can look back at it and find some in the future. Also check to see if your system is low on memory during these times.
To be clear: I've been doing this for 10 years. Never bumped into this problem. But we don't have many clients who use GSM. Which brings up another interesting point: WHY GSM?