I had this exact same problem, I found the bearings that run in the vee grooves on the column had all but seized up. I was lucky enough that cleaning the grease out and putting fresh in cured the problem completely.
It sounds like the focusing gear/rack have sat in the same position for a while and the plastic (?) focusing gear has taken a set.
Replacing the focusing gear will fix the problem, but the problem should also fix itself over time as the gear returns to its former shape.
If the problem is really annoying you may want to slip the gear off, rotate it a few teeth and slip it back on. This won't fix the problem but the flat spot will be somewhere else where -hopefully- it doesn't matter.
I've been off-line a couple of days... during part of which I turned the beast around (HEAYV) and pulled a few covers off with a screwdriver - and found some crudded up bearings and cables. I spent a while cleaning bearings and the V-groove in which they run up the girder, wiping off the cables and generally tidying things up inside the column.
Now she runs up and down and focusses smoothly. Wow.
Mick, I can see why you feel this beast is nearly indestructible. Amazingly well built, or at least compared the the various Beseler and Omega 4x5 enlargers I've worked with. The big Durst L1200 (that now lives in my home darkroom) is similarly well built but with, seemingly, a bit more finesse and polish(?). (the Ferarri Factor?)
Anyhow... thanks to all of you for your suggestions, the DeVere is working very nicely and one of this year's new students has already declared it is "her" enlarger for the year.
Oh, by the way, did I mention it was free? (a take it now or it's in the bin thing)
OBTW... Anybody know what the plastic screws with the spring on it located at the back, behind the baseboard assembly, in line with the axis of both the focus and height adjustment axles are? (a preload or positioning device for the axles?)