  We report on spiking dynamics of frequency up-converted emission at 420 nm
generated on the 6P3/2-5S1/2 transition in Rb vapour two-photon excited to the
5D5/2 level with laser light at 780 and 776 nm. The spike duration is less than
the natural lifetime of any excited level involved in the interaction with both
continuous and pulsed pump radiation. The spikes at 420 nm are attributed to
temporal properties of the directional emission at 5.23 {\mu}m generated on the
population inverted 5D5/2-6P3/2 transition. A link between the spiking regime
and cooperative effects is discussed. We suggest that the observed stochastic
behaviour is due to the quantum-mechanical nature of the cooperative effects
rather than random fluctuation of the applied laser fields.
