About two weeks ago, the display on my Jennair wall oven became invisible when I heated my oven up to 500 degrees. The oven and timers were all still functioning, I just couldn't see them anymore. When the oven cooled down the display was visible again - although it was somewhat dim. This pattern would repeat whenever oven was used. Earlier this morning the display was visible but now it has disappeared again without the oven being on, but again, timer is still working. Any suggestions?