Randomiser

Fearing the vengeance of the Black Guardian following the quest for the Key to Time, the Doctor installed a device known as the Randomiser in his TARDIS' control systems. Reasoning that if he himself did not know where he was going, his enemy wouldn't either (and would thus be unable to pursue or act against him). (DW: The Armageddon Factor)

The Randomiser, when activated, scrambled the TARDIS' co-ordinate settings giving the Doctor even less control than usual over the destination of his ship. As the name implies, using the Randomiser could send the TARDIS to any point in space and time.

Given the size of the universe and the fact that the TARDIS still showed a marked tendency to materialise on habitable planets, rather than in inter-galactic deep space, it appears that the process was not completely random. The Doctor's first two on-screen uses of the device both sent him to planets he had previously visited on numerous occasions (Skaro and Earth).

The Doctor frequently chose to over-ride the device when it became necessary to visit a specific time or place. (DW: City of Death, The Creature from the Pit)

Dialogue in City of Death could be interpreted to mean the Randomiser could be partially engaged, so the Doctor would visit a particular place but at a random point in history - such as in the case of the visit to Paris, where the Doctor complains that 1979 is not a very good year, but then accepts that he can't expect the Randomiser to have very good taste. The implication is that Paris was the Doctor's choice of destination but the actual time period was up to the Randomiser.

In the end the Doctor chose to take his chances with the Black Guardian, removing the Randomiser as its components were needed within the Tachyon Recreation Generator on Argolis. He made no effort to replace the device despite his earlier concerns. (DW: The Leisure Hive)