We have observed, in a commercial aqueous polyacrylic ester dispersion, a shear thickening phenomenon that exhibits time-dependent rheological behavior, The critical shear rate gamma(c) for the shear thickening transition varies with volume fraction, temperature, pH, and particle size distribution, in a manner which indicates that the phenomenon is associated with a reversible shear-induced colloidal order-disorder transition. However, we find that, in the shear thickening region, the rheology may also evolve with time. Our observations suggest that the time-dependence is caused by the temporary formation of particle clusters at high shear rates.