Kenmore Oven Model# 790.95889300
We set it on preheat to 400 degrees, when it beeps, the inside gauge we put in indicates actual temperature is 300 degrees, so then set it at 475, and it then reads internal temp at 375-400, then we put the food in, and can't leave it set at 475, because it starts rising on it's own and gets too hot, it will vary and go up to 410 and up to 450 degrees on it's own. What should I start with replacing?

The temperature control is most likely failing or just out of calibration. I have calibrated many commercial ovens but never a residential unit.

Thank you, is there a temperature control part that I would buy?

Check with an appliance repair shop or on-line as to your specific manufacturer. Please note that while I do repair commercial equipment I do not work on residential. I base my answer to your question on my commercial experience.

Okay, thank you for your help.

No worries. Hope you get it dialed in.