ActiveSync 4.0 is not stable. Update to
ActiveSync 4.2 and see if that makes a difference.
Also make sure you've plugged your pocket pc directly into AC power and the USB cable is plugged directly into a USB port on your computer (don't use a USB hub). Remove any installed storage cards and turn of any power saving options on your computer. Disable your firewall and antivirus software before you start running the update.