Background: 1964 ranch on slab with cast iron DWV. My wife & I have been in the house 3 years. This problem has occured once before.

Problem: Kitchen sink drainage slows to a crawl then stops. I notice the problem getting worse over a period of about a week or 10 days.

What I've Done: Removed the trap bottom and snaked from the trap to the stack, a distance of about 5'. Didn't feel any resistance This stack vents only the kitchen sink. Removed the cleanout on the exterior wall and snaked an additional 20'. While using this snake I felt resistance after going down the 2' vertical drop, the resistance continued for about 12'. When I retracted this snake the tip was covered in a hard mud, not like the soft greasy goo I'm familiar with. Up at the roof vent I snaked with my 50' coil and ran water through the sink while I retracted the snake. It was fairly smooth sailing with that.

The stumper: The drain will now take flow all day long from our 2.5 gal/min faucet. When I fill the sink up and pull the stopper the first 3 gallons flies down without problems but then the flow slows. Better than before but still not right. The laundry room and powder room have never caused any problems. Both of these rooms are further away from the street (city sewer) and both of these rooms are 10'-20' away from the kitchen sink.

Any opinions as to what's going on or what to try next?

Thanks so much.