[en] The dynamic behaviour of the L-H transition in ASDEX-Upgrade is described. We focus on the periodic L-H-L transitions known as 'dithering H-mode' and show that there is an intrinsic timescale in this process which is characteristic for the L-H transition. We give a tentative explanation for the phenomenon using an extension of Itoh's model for the L-H transition and find that this simple bifurcation model can qualitatively explain many of the experimental observations. According to our model, the dithering cycles are a limit cycle oscillation due to a difference in the response of the system to a rise in either T or n. The implications for H-mode theories are discussed. (orig.)