is there any chance there's a bit of fuel drop out at low rpm/idle due to stuff all intake flow and that 180 turn? that'd easily enough lead to errative a/f readings.

I don't know whether to suggest this or not, but you could try passing an oxy torch over teh manifold (engine off, and insulation wrap removed) and heating the runners up and then starting it and seeing if it affected the idle, and if it then started to deteriorate as teh runners cooled back down. I'm not suggesting you get them glowing white hot or anything mind you!