I experienced the same thing with essentialy the same set up. If it runs okay under load then a vacuum leak is a likely suspect. In my case it was a vacuum leak at a junction of one of the myriad smog control lines. Get a can of spray carb cleaner and with the engine at idle (or adjusted slightly above if you need to to keep it running) spray a bit at the various vacuum connections and lines. If you hit the leak area the idle will smooth out for a short time. Check the vacuum line to the distributor.
cct
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