We have studied the Coulomb anomaly in the conductance of a phase-coherent disordered 2-dimensional electron gas. Its average magnitude of delta G(EEI)approximate to - 0.3 e(2)/h is obtained by applying a bias voltage to suppress the Coulomb anomaly. This Coulomb anomaly is suppressed by a parallel magnetic field in contrast to theoretical predictions. Surprisingly, the magnitude of this Coulomb anomaly exhibits reproducible fluctuations in perpendicular magnetic fields on a magnetic field scale much larger than that for universal conductance fluctuations. (C) 1998 Elsevier Science Ltd. All rights reserved.