Many historians now believe that the tales of Messalina's sexual excesses are either exaggerated or fabricated posthumously in order to discredit her and justify her execution. The main sources are Tacitus, Suetonius, Juvenal and Pliny the Elder writing between 30 and 70 years after her death.
She did cheat on Claudius, whom she had been forced to marry by Caligula, and she did marry her lover Gaius Silius while Claudius was out of town. This may have been part of a failed coupe attempt. However, her motive may have been more about protecting her son Britannicus and his succession than a grab for power. Claudius' niece, Agrippina, was maneuvering to have her son, Nero named as successor. Of course, Agrippina became Claudius' second wife, had Nero named successor, poisoned Claudius and had Britannicus murdered. Interestingly, a memoir by Agrippina is one of the few sources Tacitus cites by name....though he may have made that up.