A conveyor belt can be adjusted to move with unnoticeable speed ripples; When loads it with a few heavy boxes, the belt speed is becoming unstable. Once the load is removed, the belt speed returns to its normality. I think this simple analogy answers all your questions. However, the complication is that the stock market is not a linear time-invariant system, perhaps , its natural frequency drifts from time to time, therefore, its resonant characteristic to an external driving force is drifting too. That leaves a few dominant cycles, for example, the Four year Presidential Cycle still persists with relative consistency. The following chart showed 3 complete different chart patterns during the time period 1994 to 1996. In between December 1994 through December 1995 it was an outright bull, all short term cycles were nearly vanished, and 1994's and 1996's exhibited complete different scripts, each one was at its own uniqueness. Regarding the cycle, here is a Mark Twain quote "By the Law of Periodical Repetition, everything which has happened once must happen again and again and again - and not capriciously, but at regular periods, and each thing in its own period, not another's, and each obeying its own law...." - Mark Twain http://www.bibliotecapleyades.net/ciencia/cycles/cycles01.htm