Our site has been at its current URL for about 6 months, having moved from a different URL after several years. Recently, I posted a new page to test a theory as to why our pages aren't showing up higher than they are. This new page quickly showed up #9 in the Google SERP for the targeted search term but disappeared completely from the SERP within 2 days.

This sounds similar to what some claim happen to new sites when they are put into the (alleged) sandbox, but I have never heard of it happening for a single page.

One theory we are considering is the possibility that we are in the (alleged) sandbox still, even after 5-6 months. Could it be that we are sandboxed and any new pages are quickly sandboxed as well?

Another possibility we are considering is that we are being penalized for duplicate content. Our site is completely dynamic and many pages are similar (but not the same - we just need to get around to making more varying templates). Could it be that this new page ranked well until it was caught by the duplicate content filter?