Unsteady idle down
 

My '84 944 starts and runs great. But, I've noticed when sitting still if you rev it up, when it idles back down it's unsteady. It will actually go below normal idle speed, almost stalling, and then back to normal after you've let off the accelerator. When driving, at any speed, and you let off the accelerator, it will do this 3 times in succession before it settles at idle speed. It's an automatic, so that's why at any speed when you let off it goes to idle. Any ideas?