Josephson junctions driven by RF excitation can exhibit phase locking - the inverse AC Josephson effect. This synchronization gives rise to quantized DC voltage plateaus called Shapiro steps that are the basis of modern voltage standards. While the nonlinear dynamics of conventional Josephson junctions were an intense area of research in the 1980s, this measurement technique has reemerged in the past decade in materials science as a probe for topological states of matter. In this talk, I will present measurements of graphene Josephson junctions which show unique patterns of Shapiro steps caused by the junctions RF environment which can significantly alter the phase dynamics. For a narrow range of applied RF power, a bistability emerges between V =hf/2e and V =-hf/2e voltage plateaus at 0 DC bias current. The switching rate between these plateaus shows a non-monotonic temperature dependence, which we attribute to memory effects giving rise to noise enhanced stability.