dhclient shutdown or reset
 

dhclient wlan0 will find my wireless network for about 5 minutes. Then it won't connect to the internet eventhough ifconfig shows that it still has an IP address. How can I shut down or reset dhclient? Shutdown it's PID?

/sbin/ifdown wlan0

I would release it using the command: dhclient -r
then renew with the command: dhclient wlan0

You could also use the command: ps -ax and kill it.
And you could use the command: service network restart

I would also check the nic settings for peerdns=yes
For example, on my box eth0 looks like this in /etc/sysconfig/network-scripts/ifcfg-eth0
PEERDNS=yes