If it is one of the old all metal door types the most likely problem is the rope seal around the inner door liner. It is probably quite worn and is allowing heat to escape past the door rather than keeping it within the oven cavity. The other problem associated with this older model is the fibreglass insulation beginning to break down and allowing heat to escape through the door. You can check this by simply touching the outside of the door when the oven has been on for an hour or so. It should be hot to the touch but NOT so hot as to burn you fingers. The only real solution to both of these problems is to replace the rope seals and the insulation but this requires quite a lot of fiddly dismantling and reassembling. The other BIG problem is that there are almost NO spares available as this model has not been produced for around 20 years or so. Good luck!
simtester, September 2008