We have performed a kinematically complete experiment on ionization of H2 by 75 keV proton impact leading to electrons with a speed equal to the projectile speed. By comparing cross sections measured with a coherent and an incoherent projectile beam we were able to perform a detailed analysis of interference effects. We found that the interference structure is significantly more damped than for a smaller electron energy studied previously. This damping is further increased if kinematic conditions are selected which favor a strong role of the post-collisional interaction between the scattered projectile and the electron ejected to the continuum by a preceding primary interaction with the projectile.