Strange SILO problem…

I use a 3 level silo structure for my website:

L1 = homepage
L2 = keyword page
L3 = supporting keyword pages

I used to rank top 5 on most of my keywords with L2 pages.

I'm seeing the following on some L2 keywords… rankins are slowly dropping to top 20 and than suddenly to page 5 of Google… when this happens I don't rank with L2 pages anymore but with a L3 supporting keyword page.

WTF is going on here Google? :)