We study long-term evolution of stellar bars in disk galaxies embedded in live dark matter halos. Using numerical modeling with N = 1.2 × 106 particles, we find that this evolution proceeds through recurrent dynamical instabilities and associated secular changes. We have studied the vertical buckling instability in the bar and find that it shortens the secular timescale of formation of peanut/boxy shaped bars (bulges) to a dynamical timescale. This instability has been associated with the overall weakening of the bar and dissolution of its outer half - a process which is driven by a developing chaos in the strong bar. The subsequent resumption of the bar growth leads to the secondary buckling instability which is responsible for the appearance of X-shaped bars (bulges). The timescale symmetry breaking in the bar midplane, 1 - 3 Gyr, can affect the observed bar fraction in the local universe and at intermediate redshifts, because of the decreased bar strength and the absence of offset dust lanes there.