In article <46tsgv$6av@pipe1.nyc.pipeline.com>, dengor@nyc.pipeline.com says...
>I have an old Bessler 23C (blue) enlarger, which is having trouble >holding focus during exposure. It appears that the focus gear slips. >In order to test this slippage, I have focused the image and watched it >move out of focus while I am still looking through the grain focuser. >Every now and then an image will hold focus through several exposures, >but the inconsistency is driving me crazy. This is definately not an >alignment problem. Any opinions on the source of this problem? or a >way to fix it?

This sounds familiar. There is another possibility if you are using
the condensers instead of a "cold-light" head: negative popping. Drove me batts, until I put two (2!) heat absorbing glasses (available from Beselar) above the condensers (The 4x5 Beselar requires only one, for some reason). You can check this by waiting for a while with the light on before focusing, then continue to check for a while without turning off the light. Check again after the light has been off a while. If
the negative is not in focus, but shifts toward correct focus as the light stays on, you have found your culprit!.
Hope this helps.