Martin, I read this after dinner, before going to develop some film.
After the film was developed I put a 4x5 neg in and using my 150 f5.6 Componon S I checked out your situation on my own enlarger.
Firstly I checked it using my Peak grain focus finder, which is about the best in the business, then I did the same thing with my Paterson (small) grain focus finder.
There was a noticeable difference between the two, but this is mainly the ability to see slightly more stopped down with the Peak, than the Paterson.
What I found after careful evaluation was that using FP4+ film, set to enlarge onto a 12x16 sheet of paper, I was able to see grain wide open. I then stopped down in ½ stops, with my eye constantly at the eyepiece. I noted that after 1 stop it looked ever so slightly that the focus had shifted, at 2 stops a poofteenth more looking out of focus, 1 more stop to f16 and my eyes just cannot see well enough to figure out what I'm seeing. This was more or less the same for the Paterson, except f11 was it on that unit.
The thing though, whilst at first it looked as though it was going out of focus, it wasn't, could your problem be the message your eye is sending to your brain, it certainly at first seemed like that to me.
Then a few times I clicked the lens open, yes it was in focus, that is, I could see the grain. I did this with both grain focusing units, they both did the same thing.
I then focused at f11, which is the darkest I can see grain at, I then opened the lens in ½ stops and watched the previous thing happen, but in reverse.
There is of course a bit of a thing happening with our eyes, they really don't see too well as the light level drops, you do have good eyesight I assume? A 150 lens is usually a reasonable amount harder to focus carefully as the enlargement is usually smaller, therefore the grain is harder to see. The light on the enlarger has to work at it's maximum spread, usually making it ½ a stop darker before you start.
As you have had the lens for some time, am I right in assuming you weren't happy with what you were producing with it?