Mini2440 WinCE 5.0 - Network connection lost after some time

I'm working on a Datalogger device developed with Mini2440.
The software read data from sensors by serial com port and store it in the
SD card memory.

The device is connected to the network and the user can access with FTP to
read and download the stored data.

MY PROBLEM IS that after some hours the Mini2440 device lost its connection
and I can't access via network (ftp, telnet, ecc).

May you help me?