We've had a plumber use a snake to make sure nothing is lodged in the "S" of the toilet. He said it could be a bird trapped in the vent that exits the roof, and that if we bought a new super-dooper water-pressure toilet from them ($900 installed) they'd run a snake up towards the rood and down towards the sewer from opening in the bathroom, at no extra charge. He lifted the tank lid to look inside, and noted that some bubbles were coming up into the tank but did not explain the significance of this symptom. What do bubbles coming up into the tank point to? Is the toilet not seated properly on the lead pipe in the floor?