I have an '84 and it has Bosch Fuel Injection. I assume that you do too. My car is stock and I was cleaning up and repainting the air cleaner box. After I reinstalled it I started up and it ran like yours is described. Then upon closer examination I discovered that i forgot to re plug in the connector for the Box. I have no Idea what it does, (my guess is that it regulated the flow of air to the system) So when I plugged it back in it worked great, just like before. Hope this helps. Maybe someone else will reply and tell me what the heck that plug does.
Ray Kiszely'84 Spider Veloce