Ok- this is a trivial issue, but it bugs me. The clock sometimes works, but mostly not. I was driving yesterday, and set the clock. It kept time for 20 minutes then stopped.

Is there something I should lube in the clock? Do these always die, or were they pretty good in general. I just want to know if it is worth my time to mess with it- maybe they are just a bad design?

The car did sit in a barn for 15 years un driven, so I guess I can cut it some slack.

i'd guess a wiring problem, and usually it's a grounding one. the ground points can be cleaned and remade, and you can add more. here is a great writeup about a supplemental ground strap for the cluster:

https://community.mercedes.me/media/files/forum_attachment/13/13069836-ea51-3987-549b-e9615ea290c6-cluster-ground-install-guide-rev-a1.pdf

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'77 240D, 504H, OM617.952, etc.

If you are going to pull the cluster to add grounds, now might be a good time to do some preventative maintenance on the clock by replacing a pair of capacitors that are prone to failure. I doubt this is the root cause of your problem, but it will hopefully prevent having to pull the cluster again if the capacitors decide to fail. I think parts are about 4$ at RadioShack if you have the tools necessary already. You'll need to be fairly competent soldering on circuit boards though.
Here's a good guide that I followed:
Repairing Non-Functioning Mercedes-Benz Clocks

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'85 300D - federal spec, built in late 84. 85 300D Complete AC System Rebuild