Gas oven not heating to temp

Question

My gas oven is not heating to tempature. Example:I turn the oven temp to 400 degrees, to cook something that should take 20 mintues to cook will take 60 minutes. The pilot light is on and the flame is blue. I've used two different propane tanks so that's not my problem. Anyone have any ideas?Same problem here! I have to turn oven to 550 and maybe it'll get to 350...MAYBE! The burners, both oven and broiler seem to light just fine!
I figure the temp sensor/thermostat.