Do you use fabric softener sheets in the dryer? If so, this may cause the moisture sensor to become coated and inhibit it's abiltity to sense the clothes and turn off.
Let me know I will give you more information on how to resolve.
Those sheets can coat the moisture sensor so that the dryer does not know when to stop. If you look inside the dryer (you'll have to stick you head inside) and down directly below the lint filter you will see two metal bars. Check them and clean them. They should have not residue on them.
You also should check you lint filter itself. Even if the filter looks ok, you should take it to a sink and run a little water on it to see if it lets the water thru the filter. If it does not you need to clean the filter also.