I did searches but couldn't come up with any direct hits.

I've had this problem occur several time since I bought the s4 in May. It's not very consistent and goes away for months at a time.

Sometimes it seems like it's searching for a consistent idle speed, up and down, up and down. The usual problem is that it starts to over idle, the RPM's just start climbing, like I'm softly pushing on the accelerator. Sometimes it seems like the accelerator cable is stuck on something and can't return. Sometimes it's even more of an acceleration, which might be trouble in bumper to bumper traffic, or if i'm backing out of parking spot in tight quarters. I usually blip the throttle a little and it back off. I think if I'd didn't it would just keep slowly revving till it hit the limiter.

I recently replaced my water pump and timing belt.I also replaced the stock chips with what I believe are the authority clones from Ebay.Neither did anything for this occasional problem.

Anyone experienced this or have any advice?

 

Make sure your throttle linkage is moving freely in BOTH directions and that your carpet isn't in the way of the loud pedal.

 

If the linkage isn't an issue, look for vacuum leaks. Idle speed is regulated by introducing varying amounts of bleed air into the manifold. The LH fuel injection controller and idle speed regulator provide the system to maintain proper idle speed.

For proper idle speed regulation, the LH needs to know that the throttle is closed. For 1987 and newer 928s, check the idle switch by measuring the resistance between LH connector pin 2 and 5 (chassis ground). It should be low (close to zero) with no pressure on the gas pedal. As soon as a little pressure is placed on the gas pedal, resistance should go up to 50K ohms or greater. While you're down there, you may as well check the high load portion of the switch too, from closed to about 2/3rds open, the resistance between pin 3 and 5 should be high (50K ohms or more). As the throttle nears wide open the resistance should drop to a low value. If you don't get these readings, your switch could be bad.

If these steps don't solve the problem, the idle speed regulator (ISR) is the next suspect. Many times a squirt of a cleaner into the regulator intake hose helps things out - but eventally the ISR may need replacement. The intake manifold must be removed so it would be time to replace all the hoses and spark knock detectors while you're in there.

Another item to check is the fuel pressure regulator, as it fails it leaks fuel into the intake that also can cause idle problems.

If replacing the ISR doesn't fix the idle problem, your LH could going bad. Improper idle speed regulation is one of the failure symptoms.