As you can read below, your understanding of Roman history is wide off the mark. The later Republican era, from the time of the Gracchi at the end of the 2nd century BC to the Battle of Actiumin 31 BC, was characterised by constant civil struggle and civil wars due to the inability of Republican institutions to properly govern a quickly growing territory and integrate an ever-greater diversity of citizens. Octavian, the later Augustus, brought peace and prosperity to the Roman world, restructured its institutions in accordance with the new needs of the times, and spread local participation rights in the political system outside Rome, throughtout Italian and non-Italian cities - altough he consolidated his personal power at the center at the expense of the Republic's traditional institutions.
Rome's Golden Era was therefore not, as you claim, during the Republic -and certainly not during Cicero's life- but between 30 BC and 180 AD. Peace, order, stablity, good government and expanding local participation rights were during this time the rule, not the exception. When the Empire's decline truly began, by the third century AD, the Senate wished each new emperor on accession: "May you be luckier than Augustus (31 BC-14 AD) and better than Trajan (98-117 AD)".
So, as you can see, you were only off in your analysis by about 200 years. But don't worry, I won't hold it against you - we all live and learn... 
The Columbia Encyclopedia, Sixth Edition, says:
[quote:pvk5zbl5]Augustus and the Pax Romana
Caesar's assassination brought anarchy, out of which the Second Triumvirate emerged with the rule of Octavian (later Augustus), Antony, and Lepidus. Octavian was Caesar's nephew, ward, and heir, and his true successor. At Actium (31 B.C.) he defeated Antony and Cleopatra and made the empire one. No change was made in the government, but Octavian received from the senate the title Augustus and from the people life tribuneship; this, with the governorship of all the provinces conferred by the senate, made him the real ruler. He was called imperator [commander] and princeps [leader] and is usually considered the first Roman emperor.
Augustus organized provincial government and the army, rebuilt Rome, and patronized the arts and letters. His rule began a long period (200 years) of peace, called the Pax Romana. During this time the Roman Empire was the largest it would ever be; its boundaries included Armenia, middle Mesopotamia, the Arabian desert, the Red Sea, Nubia, the Sahara, the Moroccan mountain mass, the Atlantic Ocean, the Irish Sea, Scotland, the North Sea, the Rhine, the Danube, the Black Sea, and the Caucasus. Augustus' chief additions to the empire were a strip along the North Sea W of the Elbe and part of the Danubian area.
The blessings of peace were great for the empire. The extensive system of Roman roads made transportation easier than it was again to be until the development of railroads. A postal service was developed closely tied in with the organization of the army. Commerce and industry were greatly developed, particularly by sea, over which grain ships carried food for Rome and the West from the ports of northern Africa. The Roman Empire became under Augustus one great nation. The enlarged view of the world made a great impression on Rome, where literary and artistic interests were of importance, although nearly always tending to imitation of Greece and of the East.
Augustus died A.D. 14 and was succeeded by his stepson Tiberius; his general Germanicus Caesar fought fruitlessly in Germany. Caligula, who followed, was a cruel tyrant (A.D. 37–A.D. 41); he was succeeded by Claudius I (A.D. 41–A.D. 54), who was dominated by his wives, but during his rule half of Britain was conquered (A.D. 43). In his time Thrace, Lydia, and Judaea were made Roman provinces. His stepson Nero (A.D. 54–A.D. 68 ) was an unparalleled tyrant. In his reign occurred the great fire of Rome (A.D. 64), attributed (probably falsely) to Nero; it burnt everything between the Caelian, the Palatine, and the Esquiline, but it was a boon to the city, for Nero moved the population to the right bank of the Tiber, then very thinly populated, and rebuilt the region with broader streets and great buildings.
With Nero the Julio-Claudian line ended. There was a brief struggle (see Galba; Otho; Vitellius) before Vespasian (A.D. 69–A.D. 79) became emperor. Under him his son Titus destroyed Jerusalem (A.D. 70); Titus then briefly succeeded his father. After his mild, rather benign rule, his brother Domitian (A.D. 81–A.D. 96), a despot and persecutor of Christians, gained the empire. In Domitian's reign Agricola conquered Britain almost entirely. Domitian was unsuccessful in his dealings with the Daci and finally bought them off. After Nerva came Trajan (A.D. 98–A.D. 117), one of the greatest of emperors. Trajan undertook great public works, defeated the Daci and established Roman colonies there (in what is now modern Romania), and pushed the eastern borders past Armenia and Mesopotamia.
Trajan's successor, Hadrian, withdrew Roman rule to the Euphrates and in Britain built his wall (Hadrian's Wall) to hold back the barbarians who constantly threatened that fast-developing province. He also reorganized the senate and the army. Roman armies were then seldom seen far from the boundaries of the empire, and life continued throughout the Roman world in peace and quiet. Italy was sinking into a purely provincial state, although many emperors made attempts to make it a special country. The successors of Hadrian were Antoninus Pius (138–161) and Marcus Aurelius (161–180), who ruled in what is commonly called the Golden Age of the empire.
The Empire Declines
With Commodus (180–192) the decline of the empire is usually said to have begun. The age of the Praetorians was then at hand, when the rise and fall of emperors was determined by this elite corps of soldiers. Septimius Severus (193–211) was unusually able for his period; he campaigned with success against the Parthians and against the Picts of N Britain. His son Caracalla is noteworthy for extending Roman citizenship to all free men of the empire and for the famous baths named after him.[/quote:pvk5zbl5]
