Reason for regarding parliamentary form of government as unstable:
In parliamentary form of government, many political parties contest elections, that is,the country has multi-party system. When one party fails to garner majority due to any reason, then some parties come together to form a coalition government. As every party has its own political ideologies and goals, there soon appear differences among the council of ministers, and then, of course, such a government comes close to collapse.
At present, many governments are formed with outside support. These governments, by their nature, are unstable. The parties providing outside support may withdraw it, and as a result, the government is ousted. In this way, a government working under pressure is always in an unstable mode. In this way, parliamentary form of government is regarded as an unstable government.