Modem Timeout


My software is deployed at a variety of sites with a variety of modems. At one site, I have two Wavecom Fastrack Supreme modems. After several hours of work (for values of several as 2<several<10) one or both modems ceases to respond. The failure is always in the context of an AT+CMGL command, immediately following a transmission using AT+CMGS in PDU mode.
Is there a problem trying to read incoming messages while the modem is transmitting?
In general I poll the modem up to 10 times in 1 second before declaring a timeout. Do I need to poll the modem for longer?
Any help would be greatly appreciated.