How to make the Raspberry Pi automatically restart the WiFi interface

My WiFi router sometimes goes haywire and the Pi won't notice when the WiFi connection is up again. So I wrote this little script:

#!/bin/bash                                   
                                              
TESTIP=192.168.1.1                            
                                              
ping -c4 ${TESTIP} > /dev/null                
                                              
if [ $? != 0 ]                                
then                                          
    logger -t $0 "WiFi seems down, restarting"
    ifdown --force wlan0                      
    ifup wlan0                                
else                                         
    logger -t $0 "WiFi seems up."            
fi                                                                    

You can put this script under /usr/local/bin and add the following line to the system wide /etc/crontab:

*/5 * * * * root /usr/local/bin/testwifi.sh

This will check every five minutes if the connection is still up, and restart it, if the router cannot be pinged. If you dislike all the syslog messages, you can comment them out in the script.
My corresponding /etc/network/interfaces looks like this (I uninstalled all the network managers):

auto lo                                                                                  
                                                                                         
iface lo inet loopback                                                                   
iface eth0 inet static                                                                   
        address 192.168.3.42                                                             
        netmask 255.255.255.0                                                            
                                                                                         
auto wlan0                                                                               
iface wlan0 inet dhcp                                                                    
      pre-up wpa_supplicant -Dwext -i wlan0 -c /etc/wpa_supplicant/wpa_supplicant.conf -B
      post-down killall wpa_supplicant ; rmmod 8192cu ; modprobe 8192cu                  
                                                                                         
iface default inet dhcp                                                                  

The wpa_supplicant.conf should be easy to generate, there are lots of guides on the web for this.