As a bit of a Hale Mary suggestion, try clamping the line from the manifold to the port on the distributor to make the vacuum advance function in the distributor inactive. Ignition timing affects engine speed - modern ECUs advance and retard ignition to stabilize the idle speed. If you already have some small idle speed fluctuations causing changes in manifold pressure the distributor may be picking those up and amplifying them by changing the timing which makes the speed fluctuations worse.
If clamping the vacuum line improves things, your problem may be related to the advance curve being used in the 123 distributor. You may be able to operate without the vacuum advance function with a slight sacrifice in your fuel consumption or you may be able to resolve the problem by selecting different advance setting.
Easy to do and doesn't cost anything to try.
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