clogged toilet

The toilet upstairs nearly overflows and then drains very slowly. The upstairs bowl, however, abruptly empties when I flush the toilet in the basement. What is wrong with the upstairs toilet?

Sounds to me like you have a partial clog in the line below the upstairs toilet. When you flush the downstairs toilet, it creates a suction that breaks the clog.

Yep. I had a toilet that was experiencing some line obstruction. The plunger and a short snake did nothing for it. I discovered I could sometimes use water flow in the tub to create a venturi effect and suck the toilet level down so that it would flush. I eventually had the line augured out along with the vent stack and other branches.

When you are paying someone to do such a nasty job, get everything cleared and make sure to chase all the garbage out with a lot of water in all branches before you release them. Otherwise you can end up chasing a clog around from branch to branch. The plumbers were a bit skeptical but humored me when I systematically started flushing branches...until a line that was free flowing a few minutes before (they had witnessed it) revealed that it was now completely clogged. That was when the really nasty work began, but they got it all.

You need a professional plumber that can do a complete job on your drains. Do not use chemical drain cleaners. Not only do they not work on clogs like you have, the will create a hazard for the plumber you will eventually have to call.