Wordpress SEO specialist

I am a journalist and publish an onlne magazine, built in Wordpress. Launched in Oct. 2011 and the domain name was bought last April, 2011. It uses the All in One SEO plugin.

In the last three to four weeks its position in Google has been extremely erratic for the search term 'northumberland'. For example, on May 3 it could not be found, May 4 was on page 36, May 5 not found. Despite pages of *unique and relevant content being added every day, the site is doing poorly for that search term and overall and performs inconsistently. I need this problem resolving quickly and a long term solution. Overall the site needs to perform better.

No inappropriate SEO methods have been used in the development of the site.

Please note I am not prepared to rewrite the whole site! So don't propose that as a solution. I will be willing to try a new SEO plugin, but will need to understand what level of work time would be involved on my part if this is a proposed solution.

*Some content is in the form of rewritten press releases from organisations such as English Heritage, National Trust etc.