Oven is not heating to correct temp

I have a GE Spectra XL44 (model: JGBP40WEC4WW) oven that is not heating to the correct temp. The oven had been taking more than twice as long as it should have to bake things earlier this year. In February I did some checking and figured out the igniter was not working and replaced it. That seemed to fix the the problem. This past week it has started taking really long to bake things again. I will set the temperature to 350 degrees and wait 20min and my oven thermometer reads about 310 degrees. I can put it at 450 and it reads about 410 degrees. I checked the igniter and the best I can tell it is working fine. It has 135 ohms of resistance, and seems to turn the burner off and on easily when the controls call for it. (unlike earlier this year when it wouldn’t turn the valve on) I checked the oven sensor as well. Inside the oven it looks ok and is attached well. It has 24 ohms of resistance which seems wrong to me? but I could not find what the ohms for the sensor is supposed to be? I was also trying to check the thermostat but didn’t get into the controls yet to test it and I’m not quite sure how to test it. So I’m wondering if anyone can give me an idea on troubleshooting this problem?