We have performed 3 one-dimensional full particle electromagnetic simulations of a quasi-perpendicular shock with the same Alfvén Mach number <i>M<sub>A</sub></i>=4.5, shock normal - magnetic field angle Θ<i><sub>Bn</sub></i>=87°, and ion and electron beta (particle to magnetic field pressure) of 0.05, but with different ion to electron mass ratios (<i>m<sub>i</sub>/m<sub>e</sub></i>=80, 400, 1840). At high ion beta it has been shown previously that the shock is steady. At low ion beta, as in the present simulations, the shock periodically reforms itself. However, whereas at unrealistically low mass ratios the reformation is due to accumulation of specularly reflected particles at the upstream edge of the foot, at the realistic mass ratio the modified two-stream instability between the incoming solar wind ions and solar wind electrons leads to ion phase mixing and thermalization. The reformation process is thereby considerably modified. At the lowest mass ratio the Buneman instability between the solar wind electrons and the reflected ions is excited, which is stabilized at higher mass ratios.