Within the last month or so I've had several sites that were ranking on the first page for their main keyword just disappear from the SERPs. I am not able to find these sites even when searching past page 15 of the SERPs. These sites are about 1.5 years old and have unique content (5 pages of unique content on each). When I do a "site:mydomain" they do show up in the index (homepage and all the pages). For one of the sites 1 did a 2500 Profile Pyramid link package about 1 month ago. The other site no other backlinking except for blog comments done about 4 months ago. So what exactly has happened to these sites? Are they sandboxed or what? Should I just build more links, add more content, or both?