Not sure if we're seeing something similar on one site, but ours is a much longer cycle - around two weeks.
The first week will have several hundred visitors from google per day, the second will have nothing, with the exception of two long tail keywords which generate the only google traffic.
Initially we thought we had a penalty, or at least an automatic filter of some sort, but it seems odd that all but two keywords disappear completely.
On the good weeks, our traffic comes from a few 'big' terms (about 40% of overall traffic) and the rest from a few referrals from each of 100+ phrases.
Edit: just an additional detail - the exact cycle is 7 days of normal traffic then six days of very low traffic and then it repeats.