Is there a probabilistic law behind the economic cycles theory?

1929, 1987, 1997 and 2008... What comes to mind when you remember that key years? For sure, you remember the big financial crash, the second financial crisis from USA, the Asiatic financial markets crash and the last financial crisis symbolized by Lemon Brothers bankruptcy (every body knows that the real root cause related to that crisis is the sub-primes crisis which had been triggered by the US real estate market in 2007)....
I read some papers and learned that some eminent economists have foretold the coming financial crisis (I`m talking about the financial crisis not the economic crisis, both concepts are different) for this year... Did you notice why? Did you see any logic behind the above progression? If you have eagle eyes, you certainly noticed that the three last financial crises dates follow approximately an arithmetical progression. In fact, we had that last 29 years a stock market crash at least every 8 years, I mean 1987, 1997 and 2008. So, since 2008+8= 2016, I do believe that specialists based their prediction on that analysis. However, if this is accurate, why we did not have any stock market crash between 1933 and 1987? (1971-1973 don`t deal with any financial crisis, but, with the first oil shock)...
So, I will appreciate if anybody could help to demonstrate the fundamentals behind the economic cycle theory
NB: For your information, the solution should be mathematically logic and similar to the cycloid issue raised by B. Pascal few centuries ago. If not, a significant change in the nature of the real worldwide economy that last decades should be studied. .

Thank you
Le Tirshata