Internal crisis of the Roman Republic before the Gracchi

Question

The beginning of the Roman Republic is traditionally dated to 509 BC, when the Roman king Tarquinius Superbus was overthrown.

Wikipedia article about the Crisis of the Roman Republic states that political instability and social unrest began with Tiberius Gracchus in 134 BC, and his proposal of land reforms.

Between the death of Tarquinius in 495 BC and the election of Gracchus as tribune in 134 BC, was there any internal political crisis serious enough to menace the Roman Republic?

Besides the outbreak of the First Servile War in 135 BC, that is.

Answer 1

The Conflict of the Orders or Struggle of the Orders affected Roman politics from 494 BC to 287 BC, or 207 years off and on.

Some or all the Plebeians sought political equality with the Patricians.

One tactic used by the Plebeians was the Secessio plebis in which all the Plebeians withdrew from Rome, bringing business to a standstill, used perhaps as many as five times between 494 and 287 BC.

So yes, the Roman Republic did experience other periods of political unrest. It is conceivable that if the Patricians did not agree to enough of the terms of the Plebeians during each and every Secessio plebis the Roman Republic would have fallen, so the Conflict of the Orders can be said to have threatened the Republic.

Answer 2

A recent book review by Ayelet Haimson Lushkov has some very insightful observations on this very topic:

[W]hile we know very little about the middle republic in comparison to the better documented period after 133 B.C.; what we do know suggests that the problems of the late republic were built into the system almost from its foundation. People like Scipio Africanus and Flamininus stretched the limits of what Roman society was willing to tolerate, and while they were more or less successfully brought back to the fold, the difference between them and Sulla, Caesar, or Pompey was in degree and not in kind.

Very briefly she goes on to offer a possible theoretical explanation:

The system of aristocratic competition demanded that individuals amass ever-growing personal and political fortunes, and historical circumstances gave them the opportunities to transform political clout into a cult of personality.

Much like in some Greek city states, I believe.

Furthermore, if one is looking for (possibly apocryphal) even earlier precursors to the outsized personalities of the late republican warlords, Coriolanus and Spurius Maelius come to mind.