My Datsun 620, which has always been a great runner, is doing exactly what you're describing. Did you find out what the problem was with it? I'd love to apply whatever fix is needed to mine and keep driving it for a long time to come. It won't idle, and stutters when changing throttle position, and I have to hold on the gas to keep it running when stopped. If anyone else has fixed this malaise, let us in on the secret!