1988 w124 idles poorly only in drive gear
 

Hello, first post here! I just bought a 1988 300E to replace my 1982 300SD with 270k miles on it. It idled poorly until I did plugs, wires, distributor, and fixed a small vacuum link.

Problem is now she idles perfect in P and N, but if i put her into a drive gear she idles rather roughly and moreover the revs drop from 700 rpm in P/N to 500 rpm in D/R if i keep my foot on the brake. Anyone have a similar problem or knows what's going on?