In this paper the problem of stochastic diffusion in a reversed field pinch (RFP) plasma in the presence of mode-locking has been addressed. In order to understand its effects, different spectral shapes of the perturbing magnetic field have been compared. The importance of long-wavelength perturbations in determining the stochastic diffusion process is pointed out. Moreover, we have considered the effect on energy transport and the conclusion has been reached that, due to the high parallel thermal conduction, `local' effects are not important. A comparison of the experimental values of the energy confinement time with that deduced assuming a collisionless stochastic transport model is also given.