We have measured fully differential cross sections for ionization of He by 3 MeV proton impact. A projectile beam with a coherence length larger than typical atomic scales was prepared at the Test Storage Ring taking advantage of electron cooling. Pronounced effects due to the projectile coherence are observed which are absent for an incoherent 100MeVu -1C6+ projectile beam and the present data are in much better agreement with theory. Thus, these results represent a major advancement in resolving the so far unexplained discrepancies between experiment and theory for the 100 MeV u-1 C6+ case.