A class of transport-relevant instabilities, including Ion Temperature Gradient (ITG), Trapped Electron (TEM) and Electron Temperature Gradient (ETG) driven modes induce a heat flux which increases strongly with temperature gradient above a certain threshold of the inverse temperature gradient length 1/LT T'/T. This behaviour can be recognised experimentally in various ways: 1. At sufficiently high heat flux the ion and electron temperature gradient length is clamped at a critical value ("stiffness" of profiles), 2. the "incremental" heat transport coefficient cinc (1/n) ðq^/ðT is larger than the heat flux coefficient derived from local power balance cpb (1/n)q^/T and 3. Edge and core temperatures are proportional and for typically flat H-mode density profiles a strong relation between edge pedestal pressure and total stored energy is found. Evidence for critical gradients from all three methods has been found experimentally, e.g. in ASDEX Upgrade for ion [1, 2] and electron [3] transport channels. In JET, previous experiments with on- and off-axis deposited ICRH [4] resulted in weak variation of the transport coefficients with heat flux. The current investigation aims to extend these studies in JET by application of increased ICRH power and variation of the edge temperature at each heating power level.