We report transient sub-picosecond pump and probe experiments performed on a semiconductor microcavity into which quantum wells have been inserted. The cavity is featuring resonance and strong coupling with the exciton. It is strongly and resonantly excited. Optical amplification of the probe beam (up to a factor of 5) appears, a blueshift of the polaritons occurs once the excitation reaches the exciton saturation density. The intensity of the probe, reflected or transmitted by the sample, oscillates while the pump to probe delay is varied. The frequency of the beats increases with pump intensity, and at the highest pumping rates it exceeds by far the frequency corresponding to the unperturbed Rabi splitting. This increase of the Rabi frequency is discussed in terms of the dynamical Stark shift of the excitons. while the amplification of the probe is qualitatively described as a coherent energy exchange between pump, excitons and probe.