I think you are correct in your diagnosis. Lint builds up in the area under the lint filter, so you want to check it out and also check out the outside vent.

To get to it, take out the screws that are found in the top of the door jam. The top will now come off and you can take off the front by taking out the two screws that hold it on. Be sure to disconnect the door switch and mark how the wires go.