I don't know how much change occurred for the 993, but my 964 was surging on idle, although mostly when hot. I don't believe there are any mechanical idle adjustments to be made on the aircooled 3.6 cars.
If it helps you any, my final diagnosis was having added too much oil at my last change, which fouled the idle stability valve, airflow meter, and/or throttle body sufficiently to make the car not idle well. I removed about a half-quart of oil the hard way, cleaned as much of the intake as I could reach (without removing the throttle body from the car) with throttle body cleaner (although I used carburetor cleaner on the ISV), and when I put it back together I had no more idle gremlins. If that hadn't worked, the next likely culprits were cylinder head temperature sensors, the O2 sensor, and after that the ignition system. I had already eliminated the fuel filter as a culprit by replacing it.