Connection to airport express dies out from iTunes after 1-3 hours

I have iTunes running in Windows XP. It is connected to zyxel WLAN AP through ethernet and Airport express is connected over wlan to this Zyxel. I have two problems where worse one is that after sometime 1-3 hours iTunes looses connection to airport express(remote speakers). Only way to return is to shut down power from AE. Then streaming can be started again. it seems that Internet connection is working all the time through zyxel, so this is not probably related to wlan itself

Other problem seems to be that Itunes can find AE only after reboot. I have opened firewall, so good ideas what is wrong in these two problems.