Dynamic crack propagation due to quasistatic loading in PMMA plates with a notch is considered. Both experimental data and results of numerical simulations are presented. The crack tip position was registered using slit-scan photography. Numerical simulations were performed using the finite element method and the incubation time fracture criterion. Special attention was paid to the unstable behaviour of the crack velocity. Processing of the experimental data revealed oscillations of the crack velocity, which were qualitatively obtained using the numerical model.