There are some rubber parts in the intake manifold, underneath all the fuel-injection stuff you see under the air filter: these could be leaking, as they are prone to do when they age. Sure it's not your plug wires, etc? Anyway, those rubber parts are worth replacing at this age.
One way to confirm a leak down under: no, I can't tell you, it's too risky.
My '86 420 was a known poor idler from new; I always knew if I could get my hands on it, I could straighten it out. Now (20 years later) it idles very smoothly, and the dealer who could not fix it when new is out of business. Hope this helps/encourages you.