The phase memory time T_M characterising phase relaxation of the spin packets forming EPR lines was measured for Cu²⁺ ions in triglycine selenate, NH₃⁺CHCOO^- radical in diglycine nitrate and SeO₃^- - radical in (NH₄)₃H(SeO₄)₂ single crystals in the temperature range 4.2-300 K using electron spin echo ESE technique. The paramagnetic centres concentration was relatively low (<10¹⁸ spins/g) and the phase relaxation was dominated by nuclear spin spectral diffusion process modulated by molecular motions. The minima observed in the temperature dependence of T_M were identified as due to flipping motion of the glycine molecules, rotation of NH₃ groups as well as radical and host molecule reorientations. The low temperature T_M minima (at of about 50 K) are due to low frequency motions (t_c ~= 10^-7 s) with very low activation barriers (E_a ~= 1 kJ/mol).