Jenn-Air Range: can’t reach & hold temperatures

[mrmambo]

Hi! My Jenn-Air JES9800BAB electric downdraft range is having trouble reaching and holding the correct temperature. 

For example: I set it to bake at 450° today and it only got up to about 400°. I then set it to 550° and it finally got up to 450°.

Second example: For pizza last week I set it to 550°; it took about 45 minutes to reach temp per the display notification (beeps). I then left it to continue heating for another 30 minutes, but my standalone oven thermometer and the surface of a pizza steel only read about 500°.  I then turned it off and then immediately set it back to 550°; the display then correctly read an accurate temp of only 500°. It seems to get near 550°, but never quite makes or maintains it. 

I’ve already:

- adjusted the oven temp to be 35° above via the control panel adjustment

- replaced the oven sensor probe; it’s correctly reading 1083 Ohm at room temperature

- checked the bake element; it’s correctly reading 24.7 Ohm

- replaced the oven control panel and noticed no difference between the new one and the old one (I swapped them in and out)

- replaced the thermostat under the glass cooktop

What else could be affecting the temperature? It’s really frustrating. I’ve had this range for 15 years and it used to reach and hold heat with no problem.