Oh, and tkamiya: I'm sure that the focus column is not slipping. The Durst 138s is a beautiful machine. I routinely leave a negative in the enlarger for a weekend, and come back to make more prints a few days later and the grain is still as sharp as when I left it. I believe I can almost 100% rule that out of the equation.
Thanks for all your thoughts on the matter.[/QUOTE
I am confused again. When you say the grain is as sharp as when you left it, is this a grain sharp image by eye? If it is a grain sharp image by a focuser then the print should be pin sharp unless the negative is out of focus. Were you saying that the print focused by eye is pin sharp or just better than those focused by the grain focusers?
If the neg is out of focus then maybe the eye which cannot use grain for sharpness is able to correct as far as possible an error in focus in the negative whereas the grain focuser simply sharpens the grain and makes the print focus worse but of course cannot sharpen the lack of focus in the neg itself.
Have you tried other negs that you know have produced sharp prints in the past using the focusers?