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good morning, sorry to hear your having issues.the biggest cause of that issue is the temp sensor. although ovens can vary in temp up to 25 degrees (high or low) it sounds like your sensor is the issue. the sensor is around $11.00 at the following link. http://www.repairclinic.com/PartDetail/Oven-Sensor/WB23T10015/911009?modelNumber=JCS966KD1CC?RCAID=1000065 this sticks out of the back wall inside the oven. it plugs in. I would recommend replacing that.
Thanks for the help...I have ordered the part. Hopefully this will work.