It is hard to say if the US form of government is really less stable than any other democracy -- since the US has been around, many countries have undergone wild changes to the structure of their governments. The US system has changed as well, it is an ever-evolving process.

Not sure the US is crumbling, there are some alarming elements but then we've always had our fair share of dummies over here. Somehow we've stumbled our way into a pretty powerful position, but of course as the UK can attest, global power can evaporate quite quickly. Hypothetically, if we undergo a Rome style transition from representative government to empire, does that count as stable?