Sites not ranked anymore, but still indexed?

Over the past 6 months i've built many sites and slowly ranked them. However, several of my sites, some of which being ranked on page 1 for well over 3 or 4 months, have dropped off of any and all rankings for any search terms for which they used to show. All of these sites are still indexed, however, which is rather confusing. Is this what is called the sandbox? If so, what causes it? I have not really touched any of these sites for an extended period of time, could this be part of the cause? Lack of change or content addition?

One of these sites, notably, was in 1st position on google within the first 2 weeks. It now disappears for several MONTHS at a time, and will seldomly reappear on its page 1 position 1 rank on google for a mere TWO days, only to disappear again for months. Why does this happen? It is actually rather funny to look at my analytics account and see the traffic going from ZERo for MONTHS at a time and SPIKE UP for one or two days, only to drop back to ZERO again for months more.

Has anyone else with many sites noticed this? Why does this happen, and what can i do to fix it? Any help is appreciated, thanks.
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