Plumbing Question, toilet bubbles?

In my house when this happens it means the main sewer line is backing up and needs snaked out.

I am 100% positive the drains are connected. Its backed up somewhere, Probably right past where the two connect. In the main sewer stack of the house, there should be a cleanout cap. Remove the cap and snake it. If you still have a problem pull the toilet and snake from there. Pretty easy fix!

you say its downstairs,its more than likely the last thing on the sewer run and close to the end before it gos into a larger section of the drain

which can build back pressure and cause the bubbles,now as far as the flushing issue,ya you probably have something in the bend or where the bowl bolts to the flange,but not enough to stop it up just slow it down,if you have access to a snake try using it and see if you can clear what ever is there.

You have a drain line stoppage. When you use water in the house, it won't drain and is forcing the trapped air up through the toilet, hense the bubbles. You need to have the line snaked.

sounds liek a problem between the toilet and the washing machine downstairs lie a clogged pipe..is it iron pipe or copper? or?

how old is your house if iron it could be getting clogged and plunging wont help much if any

It sounds to me like your vent pipe may not be large enough or you may not have enough of them or maybe one of them is clogged. I think it more of a trapped air problem then a clog. If you know which of the pipes on your roof are the vents you could put some water down them using a water hose but if you don't you might want to call a plumber because you could cause some damage. Good luck!

Sounds like there is a clog, because it is draining slow that's why you get that bubbling effect. May need your drains unclogged.