I have a Real Estate website. It's approximately 5 months old. It's a CMS website purchased from a vendor. I've been working diligently, albeit part-time, to boost the site in the rankings for certain keywords by adding content including videos and blog entries, by participating in Social Media, particularly Google+, and by trying to correct problems on the site such as duplicate titles. I have about 5500 Not Founds listed in the Crawl Errors section of Google Webmasters. These are partially the result of reorganizing the site, i.e. renaming or removing pages. In particular, there are individual pages for listings that come and go frequently.
I have read on this forum and from the Google Help that Not Founds should not affect SEO. I should say that there are no current links on the site to non-existent pages. The important thing is to have 301 redirects for pages that actually exist by another name on your site and a custom 404 page for the pages that are gone and will not return. In this case, the site is so new that I don't see the need for 301 redirects; there are no links to pages that have been renamed.
The reason I am posting is that lately, even though I have been quite active on Google+ lately with posts linking back to the site that are getting comments and +1s, I am losing ground on the one key phrase that I check daily over the course of the last month. When I say losing ground, the site has been as high as 29 for this phrase and now is at 42. I understand there can be considerable fluctuation from day to day for a given keyword. Perhaps what I'm seeing is to be expected. Another fact to mention is that approximately 5k listing pages have been added over the time frame that I'm talking about. I've added approximately 10 back links over this period, all from Google.com as a result of my Google+ activities. I feel like I'm not making headway and other than these 404s I'm not sure what else could be the problem. My back links haven't changed much over this time, as mentioned. They're from forums, google.com, a few from free directories, one from my employer.
Any help appreciated