Relaxation processes of bulk poly(isobornyl methacrylate) (PIMA) and its 50% random copolymer with poly(methyl methacrylate) (PMMA) have been studied in the glass transition temperature region by means of photon correlation spectroscopy (PCS) and dielectric relaxation spectroscopy (DS). The experimental results were analyzed in a consistent way employing an inverse Laplace transform analysis of the time correlation function of density fluctuations and the dielectric relaxation spectroscopic data utilizing a recently developed algorithm. For bulk PIMA, the time correlation function clearly shows two distinct relaxation modes, whereas dielectric relaxation is dominated by the alpha-relaxation mode. The fast process detected in PCS is assigned to be the rotation arising from the rigid isobornyl group about the C-O bond. In the 50% random copolymer, DS displays a beta-relaxation and a slower alpha-relaxation process, in agreement with the PCS results. Moreover, in PCS the time correlation function of density fluctuations shows strong evidence of a slow relaxation mode, which exhibits a narrow distribution of relaxation times. This mode is tentatively assigned to be arising probably from composition fluctuations in the copolymer. The molecular origin of these modes is discussed.