DC 9.62 PPP "PPPLINK: sendpacket no buf avail" error

I’m using a Rabbit LP3500 to send data to a satellite modem using PPP over serial. I’ve been successful using ifconfig() with sendexpect to start and end a data call. However, sometimes (it appears to be very random), the link fails in a loop with the verbose message “PPPLINK: sendpacket no buf avail”. Any ideas on what might be causing this? Thanks.