My 87 in stock form would alway shave an off idle. Did all the same and went nuts over it. Even weirder, at around 40 - 55*F it ran flawlessly, no vibration, and colder or warmer, junk
2 areas of note that made improvement:
1 - CTS & IAT sensors made a difference
2 - There is a vacuulm T that branches for the vacuum canister on the firewall and other towward the transaxle (cruise if I remeber right). Every connection on that whole vacuum line would get pretty flimsy very quicly due to the location (engine/trans/exhaust crassover all being there. Especaill pay note to the 'nipple' on the vacuum canister.