You are not getting enough cooling or the fan is blocked up.
You can track the fan speed and temperature in the airport utility by going to
Here is the fun bit.. there is no way to clean the fan except by doing a tear down of the AE. And if you have warranty or applecare.. then that will break it.
So take it back to apple for replacement. So they can clean the fan and sell it again in the refurbished store.
If you are out of warranty it might be possible to still buy some.. if the AE is less than 2years old.. not many would be over that.. buy applecare on an ipad or apple tv which will cover it..