Mark Crabtree is right: I should have posted my specs a bit clearer off the bat.
Light source is tungsten - one of the Durst Atlas bulbs made for the 138s. I use all sorts of paper. This test was done on Ilford Warmtone Multigrade RC. I did not use any filtration. When I do use filters, I have an Ilford set of gels. I never focus with filters in (that included the red safelight filter). My lens is one of the newer Schneider Componon-S 80mm with the focus lever on it. I focus wide open. In the future I will focus at a working aperture.
The reason I conducted this test with the lens wide open was that I was hoping it would show a worst case scenario insofar as focus was confused.
The filtration issue - blue filter, yellow filter - doesn't quite make sense to me. Here's the thing: when I focus with my eyes, it's very sharp on the easel. When I focus with the grain focuser, it's sharp but not *very* sharp on the easel. The prints I made on RC paper confirmed that to me. At the moment, this is all I'm worried about.
And pentaxuser: to avoid confusion - if I focus the image with a grain focuser and the grain is sharp (or it appears to be sharp!), and I leave the enlarger for a weekend and come back and check the focus with the grain focuser again, it is still sharp. What I'm saying is that the focus column on the Durst 138s doesn't budge. It's got locking focus and it does its job very well.