The NMR spin-lattice relaxation time at three Larmor frequencies - v(L) = 28 MHz, 1.235 MHz and 48 kHz - in the isotropic phase of butylcyano-phenylcyclohexane has been measured in the temperature range close to the isotropic-nematic phase transition. Spin-lattice relaxation mechanisms provided by both individual reorientational fluctuations and fluctuations of the local director can be distinguished at the different Larmor frequency scales. We conclude that the Landau-De Gennes theory for the weak first order nematic-isotropic phase transition, together the low frequency approximation can be applied if the experiment is carried out at Larmor frequencies in the kHz range.