I had a similar problem with my previous double oven, the smaller oven also incorporated a grill element. The thermostat used to cut the grill element off so it never stayed at its maximum temperature, impossible to grill anything.
I got in touch with the manufacturer (could have been an electrician, it's a long time ago) who came out and said the thermostat was faulty and changed it. No further problems, grill stayed on full unless I turned it down myself using the grill control knob.