Hi & Welcome to Linux Questions.
I'm not an expert with WiFi.
It sounds like your Network Manager isn't establishing a connection at times or possibly the network interfaces file may not be configured correctly. Take a look at your /etc/network/interfaces file and see if Wired is commented out.
What distribution are you running?
Is the eeepc a Asus laptop?
What WiFi card do you have? Run: lspci | grep -i network
to find out:-
This thread might help.