Why does our toilet keep blocking and then slowly draining?
The one time this happened to me it turned out that my 1.5-yr-old had flushed a quarter-turn piece of his hamster run into the toilet.  

The snake went through it and we couldn't find any obstruction.  

So we decided to replace the toilet.  

As soon as it was off the floor we turned it over and there it was.  A nice piece of blue curved plastic.

It was wedged in such a way that the snake made it pivot to go through, but water flow would catch in it and it would turn back to block the flow.

So the answer in our case was to remove the toilet and give it a thorough examination.