Starting Problem

Hi I have a 1986 Audi 90 and am having some serious issues with the start up. The engine turn over is good, but every few seconds it gives a Pow! like it is missing a stroke. After a while it will start and seems to idle fine for about 10-15 minutes. If I touch the accelerator the RPM drop to about 100 and the dash lights come one, then seems to recover goes back up to 850-900 and stays. If I feather the accelerator ever so gently it will get past 1000 and then it it smooth as. Auto transmission, when I put it in drive she stutters, coughs runs rough as guts and then dies.

This is all repeated every time. Any ideas where I start to look? Plugs are new, oil is topped. Engine has never given me any problems until now. Where to look first?