Random Stutter Problem

[Jubykiwi]

I have a beautiful 1986 911 Targa which has a vexing erratic stutter problem. I have replaced the plugs and have replaced the DME unit.

The cars starts no problems but then randomly - sometimes at idle, sometimes under acceleration, sometime a steady speed it will start running rough like it is on 4 cylinders. Often this will last for 5-20 secs and revert to good. Other times it will simply stutter every 1 second for 5 secs-1min and then revert to good.

Any suggestions for what the issue could be or how I can test welcomed.