  By means of NMR experiment and MD computer simulation we investigate the
dynamical properties of a chloroadamantane orientationally disordered crystal.
We find a plastic-plastic dynamical transition at T_x ~ 330 K in the
pico-nanosecond regime. It is interpreted as the rotational analogue of the
Goldstein crossing temperature between quasi-free diffusion and activated
regime predicted in liquids. Below T_x, NMR experimental data are well
described by a Frenkel model corresponding to a strongly anisotropic motion. At
higher temperatures, a drastic deviation is observed toward quasi-isotropic
rotational diffusion. Close to T_x, we observe that two-step relaxations
emerge. An interpretation which is based on the present study of a specific
heat anomaly detected by a recent calorimetric experiment is proposed.
