You need to install two software packages to make the internet connectivity work consistently without breaking. They are wvdial and gnome-ppp (The front-end tool to configure wvdial).
1) Install wvdial package
Open terminal,(alt+ctrl+t) and type the following command
sudo apt-get install wvdial
2) Install gnome-ppp
sudo apt-get install gnome-ppp
3) Open gnome-ppp from the terminal.
4)You have to enter your docomo 10-digit mobile number in the Username & your password. Phone=
*99***1# or if that doesnt work enter phone=
*99#. Then press the Setup button below.
5) In the Setup window below, click the “Detect” button to identify the device ID of the USB port in which you have inserted your USB modem. The tool would automatically detect the Device, Type and Speed of your USB modem. Once it has deteced, please the “Close” button below.
Once this is done click on Connect!!!
You could also download and install ixconn. You can get it here.