The alternate (stick-slip) cracking phenomenon in Poly(methyl methacrylate) (PMMA) was investigated using high-speed imaging and digital image correlation (DIC). PMMA is known to show a great variety of fracture behaviors by even small changes in loading conditions. With TDCB-shaped samples and under a range of constant extension rates, the phenomenon of alternate cracking is observed. Here, loops of successive quasi-static and dynamic crack propagation are found within a single fracture experiment suggesting a 'forbidden' velocity regime. For the first time, such material/structural cyclic fracture behavior is examined through the lens of linear elastic fracture mechanics (LEFM) by using in-situ High-Speed (HS) DIC. Energy release rates and crack velocities during fracture experiments are derived from full-field measurements using Williams' series expansion. Fracture surfaces of post-mortem samples have been systematically analyzed using optical microscopy. The investigation of the actual limits of the 'forbidden' velocity regime in terms of critical velocity and energy release rate in relation to post-mortem crack length features is achieved by holistic experimental data on alternate cracking. This work provides key experimental data regarding the improved understanding of a unified theoretical framework of crack instabilities.