Try setting a static IP address on your USB interface. Edit the The Network Manager is infamous for causing problems.
"/etc/network/interfaces" is your config file
https://wiki.debian.org/NetworkConfi...rnet_Interface
If your static settings remain when you reboot, restart the network service, unplug the USB, or go into sleep, then you know it is DHCP. If you have problems with Screen saver after this, then there may be something to dive into.