The dynamics induced by the existence of different timescales in a system is explored, in the context of a model composed of activation and signalling modules regulated by a slowly evolving process, such as some particular protein-protein interactions or genetic-like dynamics. It is shown that slowly varying regulation patterns can induce rapid changes in the steady states of the (fast varying) signal transduction pathway, and lead to sustained oscillations. These results are illustrated by a reduced model of the Cdc2-cyclin B cell cycle oscillator. Using available experimental data, parameters of the model are estimated and found to agree with the requirements for a mechanism for oscillatory behaviour arising from coupling fast and slow processes.