My oven will not hold the temp once it reaches the set temperature. If I set it for 375 it reaches 375. In about 20 minutes I will go back and while the setting still says 375 if I turn the oven off and right back on again for the 375 setting it will be down to around 340 and needs to reheat to bring back to 375. This is without touching the door. Any ideas on what we should check/replace to get it working properly? Thanks