Instabilities and strong spatiotemporal fluctuations in out-of-plane demagnetized stripe domain patterns of a Pt/Co(0.5 nm)/Pt film designed by Ga+ ion irradiation were visualized by polar magneto-optical Kerr effect microscopy and studied in the spin reorientation transition temperature region. A two-dimensional-stripe melting transition was evidenced before reaching the in-plane spin state. Slow dynamics were driven by short-range instabilities on magnetic defects which trigger long-range transverse wall fluctuations. The spatial and temporal behavior of stripelike “floating” patches was investigated. Dynamics cannot be simply interpreted by models previously proposed for liquid crystals.