faulty grill or not?

Zanussi ZKS5010W1 I recently purchased this cooker with separate grill. When I first used the (variable) grill, I turned I on full and found that after 5 mins, the element which was red hot, seemed to cool down and turn grey/black again only producing enough heat to keep foo warm. Long story cut short - service engineer had a look and the cooker was eventually replaced today. The new grill has just done the same thing. Should the element for the grill not stay red hot when on full?