We report on incoherent neutron scattering results in the glass-forming liquid di-2-ethylhexyl phthalate (DOP) (Tg=184 K). We are particularly interested in the temperature dependence of (i) the elastic scattered intensity, (ii) the mean-square displacement and (iii) the shear viscosity. Our neutron scattering results are internally consistent and compatible with the existence of a dynamic instability at temperatures above Tg, as predicted by the recent mode coupling theory. This is in agreement with recent independent measurements on the same liquid from dielectric relaxation and depolarized Rayleigh scattering. However, an alternative approach  based on free-volume arguments shows no discontinuity over a broad T-range.