I don't have a time framework for the question, but I can give some reaons for the most recent instability: 1) cartel wars: Mexico is the country through which drugs from South America reach the US, which leads to illegal activities, and impedes stability - in some places the cartels have full control over the place and the government cannot do anything 2) high corruption among the oficials, caused by poverty and lack of control - it is widely known that police can be "bough out" and this decreases stability