The Roman Republic in History

[Warmaster Horus]

Depends what kind of books you read, but in my opinion (which is that of a 17 year old fan of Rome), the problems with the Republic started when it grew too far. Ruling Italy couldn't have been that hard, and adding Sicily shouldn't have been too hard either, but once there was Iberia, north Africa, Gaul, Macedonia, Asia Minor, there was just too much.
Which is where the idea of a professional army came, and how Marius reformed the army. The problem then was that the soldiers were basically an extension of the General: he could treat them kindly, give them large bounties, let them pillage and raid, and then they would be his, literally. After that, human greed for money and power came into motion. Civil wars and eventually the time when the Senate was nearly entirely corrupt.
The Republic lasted 400+years, but the majority of that time was spent in Italy. Sicily was only added to the Republic near 250BC, and the Principate started in 27BC. You can do the math.
It was strong, because power was always divided. Consuls mean "those who go together", and that was what made the Republic a good government. The only problems arose when there was waaay too much to govern.

Of course, that's the way I see it. It's possible that Senators were always somewhat greedy, but it was relatively limited, before the Republic's major expansion.
And BTW teh1337tim, the guy that went against Parthia was Marcus Cornelius Crassus, who got owned at Carrhae by a General named Surena in 53BC (I think that's the date).