Since you are talking about a fuel distributor I am assuming your car has CIS and not D-Jet. Does it idles better or worse when the Climate Control works? This could lead to a vaccum issue.
Check the sensor plate in the air flow meter to see if it's moving freely. I suggest you replace the spark plug wires also, I had an issue in my 1980 450SEL where it would idle rough after sitting for a few days and changing the wires made a big difference.
Your car should have a vaccum valve to maintain idle when the compressor kicks in. Vaccum issues on these cars can have very strange symptoms so make sure all your vaccum systems work properly. Let me know how it goes.
1987 Jaguar XJ6