This was a pretty easy diagnosis of a broken heating element. As a precaution, we also replaced the thermistor with no luck. The reason we replaced the thermistor first is because of the symptom of graduating and fluctuating heat. In reality it was just slowly declining. Replacing the heating element in our dryer, a front loading Kenmore Elite, was a bit more complicated since we had to remove the front and back panels to properly access the heating element. The lower front panel of the dryer needs to be replaced because the element housing connection to the front support needs to be disconnected so that you can remove the heating element housing; and therefore the heating element from the housing. The rear panel, and therefore the top panel, needs to be removed so that you can easily access the wires to the heating element. Be sure to take a picture of how the wires are connected to the heating element through the housing. Once you remove the heating element housing, you can easily remove and replace the element. Be sure to wear some type of gloves so you do not get any oils on the heating element coils themselves.
Removing the front and back panels sounds like a lot. But doing so allows you to thoroughly clean the dryer as well. And if you're at the stage where you need to replace a heating element, you're probably at the point where you need to clean the unit as well.
All the bolts are 1/4 inch except for two which connect the wiring bus to the frame. You will need a ratchet wrench, an extender screwdriver that can hold the 1/4 filing to remove the bolts, pliers to easily remove the element from the housing, and a Philips head to reattach the power cord wiring assembly.
I'd say the complete job is probably an hour tops, once you know what and how to access.