Oven Door allows too much heat to escape?
It varies by model number but generally there's a spring that attaches to the door hinge. If the hinge (or other part that links to the spring) breaks then there isn't enough tension to keep the door closed. 

I had the same problem. In my case the roller broke which caused the hinge to not operate properly.

Look up your model number and look at the parts diagram to get a better sense of how your oven is set up.

If you remove the door (usually lifts straight up) you can see everything better and operate the hinges manually to see which one has a problem.