When I flush the toilet in the basement there is a big air bubble that comes up the toilet and it then drains slow; if anything else is running it almost overflows. The basement toilet was working fine for 30yrs; now all of the sudden this is happening. Could there be a clog in the drain????? The basement toilet has a vent pipe in the wall.
If there was a clog in the main drain line going out to the street, you would likely see everything in the house, or at least the basement and ground floor, being backed up or draining slow. More likely is that some animal has set up shop in the vent pipe that serves the toilet. A nest or similar made of twigs and leaves can end up being a pretty effective stopper and prevent air from entering as it's supposed to.