Welcome to WebmasterWorld Guest from 22.214.171.124
Basically, the site is a few years old, ranks well for some competitive terms (mostly just the homepage ranks, not the 100 or so content pages)... but every other day it completely disappears from the SERPs.
I've seen this with new sites, sites that just broke free from the sandbox, or sites that have had recent major changes. I've just never seen it go on this long with an established site.
Any similar experiences?