My dryer will only dry on certain settings. It

Check your vent on the outside of the house! If you do not have a lot of air passing through the vent, the dryer will overheat and can ruin the thermostats and the heating element. Clean the vent out and check to make sure the vent hose is not pinched causing a restriction. If the vent is blowing forceful air, then it could be a thermostat but I doubt it. If it does not run in auto when it's cold, chances are that your timer is faulty.