Floor drain backup - toilet water disappearing - possible Vent issues

Not sure what type of problem I have. I seem to have an issue with my floor drain a couple times a year. This may be 3 times in one week or once every 4 months. No year is the same.

So, every once in a while my floor drain will back up just on to the floor (house is 1968). I had the drains cleaned and inspected but no blockage was found to the street. The weird this is that I know this will occur because the toilet in the basement will suddenly be dry. All the water sucked out of the bowl. Not evaporation. The change me be from the morning to the afternoon.

As soon as I plunge the toilet vigorously, the drain floor drain immediately clears and I can pour an unlimited amount of water down the floor drain.

I am thinking vent issue but any help is appreciated.

Thanks,

Paul