I've been doing organic SEO for about 5 years. My bet is that your site just is not strong enough to stay at the top. It's only strong enough to stay there 1/2 the time.

I'd say just focus on new...

Just be patient. It could take up to 18 months to see top 100 ranking for your keywords.

Google likes natural backlinks. That's really all they say. A backlink from a blog is more natural when it is in the comment section and not the name section.