zanussi cooker

ZKC6020B 948904208 customer reports that the front right ring when in use after a few minutes on high will cut down to half heat. This machine uses selector switches not simmer stats and when compared with the rear left ring (same switch and element) switch positions compare identically to ring operation (elements show the same glow). I thought that the switch might be failing as it was slightly rough to turn but continues the same fault. The next obvious step is the element but this is expensive, as I have no wiring diagram to check I am at a loss to see if there is any cut out that would cause this .Is the element sensor which is part of the ring a suspect never seen this before, any ideas ?

thanks wilf