I have a Beseler 45MCRX enlarger which I bought well used about 20 years ago. It has the standard condenser light source. I recently added the geared focus drive accessory that somebody is making for it (I forget the name of the company at the moment) which basically puts about a 7:1 gear train between the focus knob and the drive pinion, vastly improving the fine focus process. I use the enlarger for everything from Minox to 4x5, but mostly for 35mm film. I use a 50mm enlarging lens (APO Rodagon, at the moment) on a recessed lens board. Virtually all of my enlargements are onto 8x10 paper, and the enlargement factor is almost never more than 10x.
That is commonly used with the Aristo VCL 4500 (and that I use) monitors the light output at the light source and adjusts times accordingly so with that timer or one like it you don't need a stabilizer. FWIW, I've always been very happy with the VCL 4500 head. Never had any operational problems and after six years of fairly.
I am generally quite satisfied with the results this enlarger provides. HOWEVER - it has an increasing tendency to drift out of focus within a few seconds to a few minutes after I focus it critically on the easel with a Peak magnifier. (I frequently make multiple prints from the same negative, so I want the focus to hold between exposures.) It drifts out of focus both 'up' and 'down' - that is, the lens stage is not simply being pulled down by gravity - sometimes I have to move it up to get it back in focus, and sometimes it has to move down. Tightening up on the lock knob on the lens stage has no effect. The lens will drift out of focus - both up and down - even when the knob is tightened as much as my fingers can do. I'm mostly using HP5 film, which dries flat as a board (unlike Delta 400, which usually dries with a curl from top to bottom.) Putting the stem of my dial thermometer on top of the negative carrier and turning on the lamp only raises the temperature at the negative from about 72 to about 82 in 5 minutes, so I can't believe that in a 30 second exposure there's enough heat to cause an otherwise flat negative to bend appreciably, and besides, over a set of 5 or 6 exposures from the same negative I'll sometimes have to adjust the focus 3 times, sometimes up and sometimes down.
What's frustrating is that there's no pattern. Sometimes the focus will stay tack sharp for a whole string of 5-10 prints, then on the next one it will be completely fuzzy by the time the first exposure is over (usually 20-30 seconds, plus time to load the paper into the easel after seting the focus and to put the exposed sheet back in the paper safe for later development - maybe 60 seconds max after setting the focus.)
I've tightened up on the nylon sliders on the back of the lens stage, to the point where the focus wheel will barely move the lens stage, and that didn't help. (I've loosened them a bit now.)
Anybody have some ideas for what to try next? (Please don't suggest I chuck it and get a Saunders or some such - I have too much invested in negative carriers and lens boards to think about that big a change at the moment.)