learning is the first derivative of knowledge. learning ebbing and flowing (assuming it never goes negative) means that knowledge is steadily increasing. knowledge ebbing and flowing means you periodically learn and then forget. but different words entirely would make the metaphor less complicated. I would say that if learning happens all at once, and then not for a while, then it happens "in fits and starts" or that knowledge is "lumpy" (but this is kind of a technical economics term, not sure how well it applies generally).
I'd go with "not well-behaved".