WIFI slowness of my Macbook Pro

My Macbook Pro (2007 edition)'s WIFI turned to be very slow recently. Speed test (http://www.speedtest.net) sometime report only 10K download while I have 15M Comcast broadband.
I struggled many days and tried different ways I could imagine, even bough a new router, problem still unresolved.

Today, I tried to check /var/log/system.log and found lots of error messages like:

Feb  9 01:10:06 li-mac-2 mDNSResponder[22]: AppendDNSNameString: Illegal empty label in name "."
Feb  9 01:10:06 li-mac-2 mDNSResponder[22]: RegisterSplitDNS: bad domain .

Then I Googled the keywords:mDNSResponder[22]: RegisterSplitDNS: bad domain . and found a hint at:
http://discussions.apple.com/message.jspa?messageID=6911574


Followed suggestion from the site, I finally managed to resolve my issue. Since the DNS error msg is gone, my MacBook Pro could do 14Mbps on wifi.

Here's summary of what I did:
1. run scutil --dns and saw something like:

DNS configuration

resolver #1
  nameserver[0] : 192.168.0.1
  order   : 200000

resolver #2
domain : .
options : pdns
timeout : 5
order : 150000

resolver #3
  domain : local
  options : mdns
  timeout : 2
  order   : 300000
...

2. Open Keychain Access
3. Click on System at left side
4. Find and delete item named DNS Key
5. Reboot
6. Problem resolved!