My GE Adora oven model J B7 10S T2SS has a problem when baking. When I set the temperature to let's say 400 degrees (it happens at other temps too.) The oven comes up to temperature and beeps and then within 10 minutes is down to 350. Within 20 minutes, the temperature has decreased further. The display still reads 400 degrees. I realized this because my food was not cooking correctly. I stopped the oven, reset it for 400 and it had to start again at a much lower temperature on the display to get up to 400. Is there something I can do to make sure it stays at the temperature where I set it?

Thank you.