Won't stay running

I have a 1985 318i ,130K. 5spd. a couple of weeks ago it started idling rough. and felt like it was running on 3 cylinders , so i replaced all the plugs and ran injector cleaner through system. that helped a little and then it got worst poor acceleration so on and so on... now it won't stay running i can start it and as long as i keep my foot on the gas it's fine. as soon as i take my foot off the gas it stalls , Any help ? thanks Jason