Starting two weeks ago our upstairs toilet will not always flush correctly. Nine out of ten times the toilet will flush fine with good suction. Then randomly it will not evacuate the bowl, filling with water to within two inches of the rim and the water just sits there and will not drain at all. It does not seem to matter what was flushed as it will do this when only water was in the bowl. I have snaked the toilet and the drain of the sink in the same bathroom and I have not found an obstruction. The downstairs toilet and sinks appear to be working correctly.