Sounds like you are not getting dhcp could be your router setting.
Type route to check if you have a root table setup the next time it happens if you have no route then go and
Try again and type ifconfig in a terminal
see if you have a consistant eth0 setting if you have then
try using a real ip number with a browser to check if you have no dhcp.
This should give you a clue why this is happening, usually because your dhcp is not being loaded from it's source.