Efficiency and quality of phase conjugation in a four-wave interaction of noncollinear diffracting light beams propagating in opposite directions are analyzed. Development of non-stationary (periodic) variations of the interaction characteristics is established. A nature of their occurrence is discussed. It is shown that the basic role in their development is played by self-action of light beams. The correctness of the obtained results is estimated with the help of spectral analysis of the characteristics of interacting waves.