The random bistable dynamics at stall of a thin symmetric airfoil is studied in the transitional regime through combined particle image velocimetry and surface pressure measurements. The statistics and dynamics of the high lift state (A) and low lift state (D) identified in the bimodal regime are analysed in detail. At high $Re$, state A presents a steady short laminar separation bubble with low fluctuations, and state D features high fluctuations, intermittent dynamics between long bubble configurations and massive separation. Broadband low-frequency energy is observed in the region near the leading edge, with a Strouhal number in agreement with the shear layer flapping phenomenon. Employing an extended version of the Multiscale proper orthogonal decomposition, we associate the broadband low-frequency energy with an up-and-down movement of the separated shear layer and a consequent shrinking expansion of the bubble. Finally, we show that the transitions between states are random and correspond to a bursting and reformation process of the laminar separation bubble. The intermittent bistable dynamics at stall is well reproduced by a stochastic model with multiplicative noise