One of the toilets in my condo, which is on the second of four floors in a 6-year-old condo building, frequently gurgles and empties by itself. Sometimes when this happens, suds appear in the toilet bowl. Also, a lesser, quieter problem in that bathroom is that the lavatory drains slowly, even after being cleared with a plunger. I have been told that the condo has iron no-hub piping, and there is a studor(sp?) vent in the cabinet under the lavatory. Anyone know what might be causing these problems?

"One of the toilets in my condo, which is on the second of four floors in a 6-year-old condo building, frequently gurgles and empties by itself. Sometimes when this happens, suds appear in the toilet bowl."
The "gurggle" and emptying of your toilet tells me that you have a blocked vent. Since washer pumps are more powerful and discharge with much more force and volume when your neighbors washer discharges some of the suds backs up into your toilet.
The lavatory drains slowly, even after being cleared with a plunger. I have been told that the condo has iron no-hub piping, and there is a studor(sp?) vent in the cabinet under the lavatory."
The cast iron No-Hub drainage pipes should not contribute to the problem. We've used them for years in our mid and high raise condos. However, substituting a AAV,(Studor Vent) for a regular vent in a condo disturbs me.
A faulty AAV could cause the slow drain problem that you have but the fact that the AAV's there at all makes me question why. We don't plumb condos that way. Good luck, Tom