Suds from the upstairs sink are coming up in the downstairs sink, also one toilet in the downstairs bathroom makes a huge bubble noise when you flush it, and will not flush completely, but needs to be flushed several times to clear it. I have had 2 plumbers and one drain cleaning company here, no one can solve this problem. Does anyone have any ideas?

It does sound like it is related to a vent. Try taking a hose and filling up the vents. Be sure to have plenty of people to watch all of the different drains that water may come up. If it comes up the vent, you will know that your vent is clogged. Then again it may be a horizontal line that has backwards grade like where a part of it has sunk down beneath the air space of the rest of the pipe if you will. You probably need to get a camera down the line.