Linksys BEFW11S4 problems-wireless connected laptop loses connection

I have this setup. A Linksys BEFSR41 DSL/router-switch connected to
my DSL modem and wired to one PC. It's been running rock solid for
two years. I added a Linksys 802.11b BEFW11S4 wireless router and
connected it to the BEFSR41 through the uplink port on the existing
router to port 1 on the wireless router. I changed the IP address on
the wireless router to 192.168.1.2 as the router connected to the DSL
modem is 192.168.1.1. On the wireless router I made it a router not a
gateway, turned off DHCP, turned on WEP, turned off broadcast of the
SSID and changed the SSID from the default. I also am filtering using
MAC address of my only wireless laptop.

All works well for long periods, then for no apparent reason or cause
I can detect yet, the laptop will report that it's lost its connection
to the wireless router and if I try to ping the wireless router from
the desktop PC, the pings time out. Pushing the reset button on the
wireless router doesn't help, but I unplug the power to the wireless
router and then plug it back in, after a minute or so, the wireless
laptop (a Gateway 200x with Centrino and WinXP) will report that its
got its connection back working.

The wireless router has firmware version 1.45.7 that I upgraded to
this morning to see if it would help. It doesn't.

Anyone with any suggestions or ideas how I can diagnose the problem?
When it works it works just great.
Larry