I am seeing this too, with just one page for just one keyword. However, for another related keyword, the exact same page is solid at #1. To me this calls into question the link devaluation theory -- if links to this page are being devalued, why doesn't it affect the ranking for the other keyword or for that matter the rankings of any other pages on the site?
To me it seems more like some sort of keyword-specific filter being turned on and off, or tuned slightly. Hit a certain threshold of some value for one particular keyword and bam, you're gone for that keyword but still fine for others. Tweaking the threshold slightly would produce this effect.