We replaced the heater coil, it works for a little while then cuts off. Could it be the thermostat?

It could be a lot of things. More than likely you need to totally tear it down and clean every part and test all safeties with an ohm meter. Here are some manuals: https://dl.dropboxusercontent.com/u/8010163/Whirlpool%20L-79%20Duet%20Sport%20Dryer%20Service%20Manual.pdf
https://dl.dropboxusercontent.com/u/8010163/How%20to%20use%20Test%20Equipment%20Service%20Manu al.pdf
You cannot half-way clean it either because all it takes is one place that is full of dirt or a dirty sensor to make it fail. Remove all parts and wash them out, except the motor. That means the door, if it has a vent in it, the filter riser. And the ducts that move air. Also, make sure to wash out the blower and the housing since all it takes is an 1/8" of dirt for you to lose 30% of the airflow. Check all safeties which are wired in series. You will probably remove a 5-gallon bucket of dirt from a dryer at least two years old.

Hi Michele, Have you made sure the vent is clear? A blocked or improper vent will cause these symptoms. Good luck

That's a good point and another thing that happens when it gets dirty. I should have mentioned to clean the vent when you clean the dryer. You have to do it all at once. Even clean the vent on the side of the house because it gets very dirty, especially if it has a screen.

I cleaned and cleared everything! It's working now. Thank you!!

I am glad you fixed it. All dryers need to be lubricated and cleaned each year using soap and water to clean it and white lithium grease on all bearings. If you will do this, the dryer will last a very long time.