Furnace fan works properly intermittently
It could be a problem with the limit switch, or the limit switch operating properly.  

If the furnace is overheating, the limit will open and cause the blower to continue until the heat is dissipated. 

It could also be that the limit is going bad, and opening when it shouldn't.  Or that the limit is opening due to overheating, but then not closing again as it should.