  We derive a spin diffusion equation for a spin-orbit coupled two-dimensional
electron gas including the Hartree-Fock field resulting from 1st order
electron-electron interactions. We find that the lifetime of the persistent
spin helix, which emerges for equal linear Rashba- and Dresselhaus spin-orbit
interactions, can be enhanced considerably for large initial spin polarizations
due to the Hartree-Fock field. The reason is a reduction of the
symmetry-breaking cubic Dresselhaus scattering rate by the Hartree-Fock field.
Also higher harmonics are generated and the polarization of the persistent spin
helix rotates out of the (Sy,Sz)-plane acquiring a finite Sx-component. This
effect becomes more pronounced, when the cubic Dresselhaus spin-orbit
interaction is large.
