We investigate the spin dynamics of electrons in quantum wells where the Rashba type of spin-orbit coupling is present in the form of random nanosize domains. We study the effect of magnetic field on the spin relaxation in these systems and show that the spatial randomness of spin-orbit coupling limits the minimum relaxation rate and leads to a Gaussian time decay of spin polarization due to memory effects. In this case the relaxation becomes faster with increase of the magnetic field in contrast to the well-known magnetic field suppression of spin relaxation.