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First, how long has this been happening?
Ok, you either have a restriction in the main sewer line or a blocked vent. Do you have a sewer cleanout in your yard? If you do, remove the cap and flush the upstairs toilet and let me know what happened.
Ok, that tells me two things. You have a venting problem and a restricted main line. Are you on city sewer or septic?
Ok. You probably have had the venting problem all along. Your sewer line was providing the venting but now is stopping up or is restricted in some way. You can rent a sewer snake to try to remove the blockage, if one exist. Take a garden hose and run lots of water down the cleanout and try to back the water up in the cleanout. This will tell you if you have a blockage that is seeping.
If the line is saging (thus creating a trap) from time and faulty installation, a sewer camera will be the only true way to find this problem. It is possible to contact your city utilities fro them to have a look at it also. (Free!!)
Hope this helps, please reply with any other questions about this.
Yes and no. To me it is an indication of a sagging line. Water can go through but no air. So when you flush the upstairs toilet, instead of the air going up a vent or out to the city some of it bubbles up in the downstairs toilet. A temporary fix is to drill a few holes in the cleanout cap that will allow the line to vent and stop the bubbling for now. I would recommend calling the city to start, then maybe have the line cameraed.