Why would a Windows 7 based PC not reconnect to the internet when both LED's on the U620L modem are green?


We have a headless (no display) PC running Windows 7 mounted on a vehicle connected to a USB620L (U620L) modem.  We use a remote connection utility to log in to this PC and monitor vehicle performance.  The vehicle is based in a area with pretty good cell coverage and we are always connect to it.  During the course of the day the vehicle can pass through areas (canyons) with poor or no service.  When this happens we lose the ability to connect to the PC (which is expected) but when the vehicle returns to a good coverage area, we still cannot establish a connection to the PC even though both LEDs on the modem are green.  When this happened we connected a monitor and mouse to the PC to try to diagnose the problem.  The PC was operational at this point but no connection could be made.  A restart of the PC would sometimes would allow a re-connection to the PC.  A complete shutdown and power up always allowed us to reconnect to the PC.  Any ideas as to the source of the re-connection problem or things we can try to always have the PC reconnect when in a good service area?