The car is old. I would want to make sure the fuel pressure is adequate in the base of the injection pump. When low the idle is not quite as smooth as the power balance seems impacted. A quick test is to close the relief valve fuel line from the relief valve. If the idle is a lot better do the relief valve spring. I was also wondering about air getting in the fuel. It has a more profound effect at idle.

Something that should be checked anyways. Many times there is a problem with a sagged fuel pressure relief valve spring with age.

Or the fuel lift pump is getting tired but I doubt this from parts of your description. It is still possible but not that probable. I think I would borrow or get the cheap harbor freight compression gage and get a baseline on that engine. If the relief valve first test show shows no difference post that as there are subsequent follow up tests. Since it starts very well cold is a good sign though.