Changing networks fail

I have a SonyEricsson MD300 mobile broadband USB modem that configures and connects fine so long as this is what I use when I first power up the desktop. I only need this however as a backup for my DSL cable connection that is about as reliable as a paper bag in a thunderstorm. :smiley: When the cable fails, I plug in the USB modem and it dials out, finds either the HSDPA or GPRS networks I am signed up for and shows connected (as per the modem colored indicator). It does not connect however no matter what I do unless I reboot first.
Any ideas?

Latest Netrunner x64 release.