Well, let's solve it......
What exactly is the modem doing after
-startup
-when online
-which koppel FW
-is there any other device connected to usb?
-after connection is lost: wait at least 5 minutes - do you get the connection again?
-when connection is lost: can you post the output of 'top'
-is any other sw installed?

how do you recognize, that "connection is lost"?
When connection is lost, can you post the result of a 'cat /etc/resolv.conf'?