Whirlpool Gas Oven not working

My Whirlpool gas oven works when it wants to. I am not real sure of the age because the stove was here when we bought the house. The stovetop works fine but the oven and broiler do not work all the time. When i initially turn the oven on it will warm up but then it will not keep the tempurature. I am not acutally sure that it warms up to the tempurature set but it gets warm. Then you can here it "clicking" and the "clicking" is more times than normal. I dont have any idea where to start…any suggestions?