I have a PR 4 website which is almost 4 year old and many keywords are already ranked. But the keyword on which I have been working right now for 3 months not getting better rank only 3 - 4 position better but when I see next day it back to the old position it is happening since I have been working on this keyword. I am building link continuously on High PR forums, Blogs, Directories and Article Site. It is also well on-page optimized. I have also unique and quality content in the website. Please advice what can I do to get top position. All advices would be appreciated.