3G Cellular stops working after a few seconds

I am testing an XBee 3G modem (XBXC) and find that it works for a while and then stops, requiring sleep/wake or airplane on/off in order to re-establish the network connection. The MY IP address is maintained and AI stays at 0, but data transmission/reception no longer works, nor do LA (DNS Lookup) commands. This happens in both transparent and API mode. I have tried both versions of the firmware 11308 and 11309.

Any ideas?