BridgeMan, when we tore out the wall the first thing I thought was it didn't look sloped at all. I checked the length of the drain pipe and it's 2' rather than the 3' I posted and at the same time I checked the pitch and it's 1/2" from the sink end to the drain end, 1/4" per foot, just as you said. When we replaced the sink, we only replaced the pipes underneath not behind the wall and I think those might be original to the house which was built in
1955. Because of that I pretty much dismissed the notion thinking the problem couldn't possibly have been going on that long.