As I understand, temperature decreased only near the edge where the fan is at. Carefully open up your laptop (Look for a tutorial, it's pretty easy) and clean the dust. I suggest lowering the air pressure, or else some parts may break. You shouldn't have waited 2 years before the first clean, that's so long.
Getting a fan wouldn't make a huge difference either, with dust on both, the motherboard and the CPU nothing will change, and if you are having RAM issues or related, you will still experience those.
> Clean off the dust
> Give your laptop some space from beneath to allow air flow, and to avoid heating of fan(s) if any, and CPU.