My son and his wife have a 2000 Saturn LS that they purchased from her Grandmother. It only has 50,000 miles on it. When it's running it runs great. But, about once a month it will not start. Starter turns over but it just won't fire up. When they try the next day it runs great again, for about a month. Any idea what the problem might be? It's got the 2.2 L 4 cylinder.