I've had the RGB745BEH1WH for about 6 years and this December it started taking longer to brown foods. I put a thermometer inside the oven and it either starts out at the correct temp and then declines or the oven display reads the desired temp and the thermometer it is off by about 75 degrees and then the temperature starts to go down the longer the oven is on but the oven display still reads the same temp. Thank you.