  We develop a theory of magneto-oscillations in photoconductivity of
multisubband two-dimensional electron systems which takes into account strong
Coulomb interaction between electrons. In the presence of a magnetic field
oriented perpendicular, internal electric fields of fluctuational origin cause
fast drift velocities of electron orbit centers which affect probabilities of
inter-subband scattering and the photoconductivity. For the electron system
formed on the liquid helium surface, internal forces are shown to suppress the
amplitude of magneto-oscillations, and change positions of magnetoconductivity
minima which evolve in zero-resistance states for high radiation power.
