I have an upstairs bathroom and downstairs bathroom in the front of the house. Both of these work fine. I have an upstairs bathroom and a kitchen directly below it in the back of the house. When I flush the toilet the kitchen sink gurgles and sometimes backs up slightly. The toilet does flush OK. This has been an ongoing problem for years. We have had several different plumbers here and they tell us we are using the wrong type of toilet paper which backs up the pipes. They clean out the line and it works fine for awhile. Another plumber told me the problem was that the drain pipe running across the basement to the main drain was running uphill and that was causing a blockage. He installed pipe supports and leveled the pipe. This worked for a few months now the problem is back. The bathtub in this bathroom drains OK, although a little slow, it does not affect the kitchen sink. This bathroom was remodeled a year ago with new toilet so I' am guessing the toilet is not the problem. Could the vent pipe coming out the roof be a problem? I'm running out of money calling plumbers without a permanent fix. Can someone out there help?