ATOT Plumbers: Toilet help needed.

[GagHalfrunt]

Also, any other bizarre drain problems? It's possible your vent stack is acting up

Try turning all of your sinks on full, then flush the toilet

The house has three full baths, the first thing I did was check the other toilets and showers to see if it was something widespread or localized. Only the single toilet was acting strangely, so I knew the problem was there. I just didn't have the diagnosis knowledge to tell a blockage apart from something more serious like a broken valve. ATOT diagnosed the problem and the simplest fix worked. As all three toilets now work properly I don't think it's a vent problem.