Foul smell in living room
Have you heard about dirty sock syndrome? Its the smell a heat pump can make if microbes start growing on the indoor coil. If you have gas heat, then never mind. I'm just going to throw out ideas to see if anything comes to mind by anyone:


a small animal like a mouse crawled in the wall and died.
There are an old drain under the slab that has a p trap that has dried out. Or possibly a sewer cleanout. Maybe a sewer pipe is cracked in the wall or in the slab.
No cat right? if a cat sprays, it stinks.
You say it not as bad by the floor. That's a good clue, keep trying to isolate it. 
Could be mold caused by a water leak or water from the roof leaking into a wall. Or any of the walls exterior walls?
I have heard of bad smells from certain kinds of bad paint batches. Did you paint recently?
Does it change with the humidity? That might be a clue.