The influence of external noise (that is fluctuation of defect production rate and heating under irradiation) on the dynamics of defect density and irradiated-crystal temperature change is examined. Time-dependence of crystal temperature and defect density according to irradiation conditions and crystal properties is one of the various stochastic processes. This is a stationary stochastic process or process which looks like auto-oscillation and Wiener's process. It is shown that regular auto-oscillation of mean values of defect density and temperature intermit with periods of irregular behavior. Characteristics of the auto-oscillations are not dependent on selection of initial conditions. The oscillation period is situated in the range 20 - 50 s. Changes of the second moments look like positive pulses, their values increase from zero up to amplitude value and then decrease again. With increase of intensity of external noise, the amplitude value begin to grow slowly, behavior of mean values do not change. When dispersion reaches the critical value, that is comparable to corresponding mean values, breakdown of the periodic regime takes place. It is characterized by sharp irregular change of all variables and significant increase of dispersion. Then the regular periodic regime is restored, amplitudes of dispersion become small and the whole process repeats again. The duration of the irregular behavior in various cases can be different. It is equal to nearly ten periods of the regular movement that continues during hundreds and thousands of its own regular periods. Regular periodic change of mean values is near to auto-oscillations of deterministic model. On the basis of these results one can conclude that the role of fluctuations of irradiation conditions becomes apparent mainly in the dynamic behavior of a system.