mobile broadband

In the last few days I have been loosing my mobile broadband connection at irregular intervals. Sometimes I can get the net for hours. At other times my computer stops being connected after only a few minutes.
When the connection breaks the “Enable mobile broadband” tick box has removed itself from the connections dialog box and I have to reboot. At which time the tick box is returned.
Alas I don’t know what the problem is, so can someone please tell me where to go looking for the answer?