Loopy Idle After About Four Minutes

[PorscheDIYer]

I opened a separate thread regarding my Idle Stabilization Valve. Thanks for the replies. Turns out that got fixed by simply replacing the part. The thing was frozen nearly shut and would not move at all. Oh well, 25 year old part. No worries. So now the car starts and idles ok. But after about four minutes, the idle gets VERY rough. Up and down between 900 rpm and nearly stalling. Driving it becomes almost impossible without keeping the gas pedal on the floor. But then, after another few minutes, voila, it fixes itself and stabilizes and drives no problem. The car is a 1989 964. Weird as heck.