We just bought a house and are experiencing a problem with our oven. When the oven is set to bake at a certain temp set point, it will turn on and start heating, but doesn't reach the actual target set point. It misses it by approximately 60 degrees. Then it will start dropping and won't turn back on until the temp drops at least 125 degrees or more. I verified this using a digital thermometer. I just tested it by setting the oven to 350 degrees. The element turned on and heated to 290, at which time the oven beeped displaying that it was at 350 even though it was not. Then I watched it drop below 220. All the while the digital readout was showing a steady 350 degrees. What should I check as being the cause of this? Thermostat? Thanks.