Join the club of the poor idling 86 420SEL's
. I have a bad idle in my 86 420SEL as well. I bought a new fuel distributor but that didnt help. Turned out to be a bad exhaust valve.
So in your diagnosis, If you find you need a new fuel distributor, let me know. I will let it go for a very reasonable price, since it is only accumulating dust on my shelf.