I heard someone use a phrase the other week that I cannot now remember that basically meant, "When this occurs, there is an abundance of it; otherwise there is a dearth." They were referring specifically to contract work: when they start picking up projects, it seems like they have more than they can handle, but when work falls off, it's tough. I think the phrase was alliterative, and I want to say the words started with S's, as in: "it's either S or S."
"When it rains, it pours" is similar, but it only covers the overabundance and not the shortage.
"After the feast comes the reckoning" is less similar, and implies that one follows from the other, which was not the case in the phrase that I'm searching for.