Why would the first few pancakes come out perfect, but only get worse from there? I have never been a pro at making pancakes. I tried again yesterday making them, using this recipe:

Ingredients
1½ cups flour (375ml)
2 eggs
600ml milk
pinch of salt

Method
Mix all the ingredients until bubbles form on the top of the mixture.  Then simply pour into a non-stick pan on medium heat. Flip the pancake once bubbles have formed on the surface.

The first 3 of 4 come out perfect: fluffy, slightly brownish and not breaking apart when flipping.  From there it only gets worse, going to big black spots, not fluffy and the spots that aren't brown are undercooked. Why is this?
Ah, the worsening pancake debacle. I know it well. We have all been there, even after training for countless hours to make the perfect soufflé at the Culinary Institute.

The pan is getting too hot. 

You should cool the pan with a quick rinse. This will also have the effect of resetting the surface, to get rid of any built-up grime or grease.

Good luck and happy cooking!