Had the exact same problem recently.ometimes it flushed fine, other times had to plunge it. Finally got tired of trying to snake it and plunging. Removed toilet, intending to thouroughly investigate. When I set it down on the ground in my backyard, out popped my long-lost toothbrush which was lodged in the trap and trapping toilet paper and other debris every so often. Question, do you have any young children in the house?