This happens upon initial startup in the AM, and smooths out after engine warmup. It seems to be amplified between 1100-1200 rpm.
Simultaneously, when the key is first turned (without cranking), there is alot of 'clicking' coming from the engine compartment. I don't know if the two are related.
The ECT sensor seemed to be an obvious choice, and a test in hot water did identify an unstable pattern when cold. Replacing this did not help the trouble.
I found a split vacuum line and replaced it-no change in status. All vacuum lines seem to be in good condition.
Autozone identified an upstream O2 code. I replaced this sensor, even though O2 is closed loop operation.
From here, the IAT seems like a good choice, though it has not thrown a code.
Any ideas appreciated!

 

IAT is something to look into, but don't rule out a coil or plug. We just had a similar problem with our s60 2.4T. The plugs were pretty worn and one coil was bad. So we put in new plugs and a coil and it ran a hundred times better.

 

The car is due for spark plugs, so I will change them.
Afterward, I'm going to run a hair dryer at the IAT & see if that produces a result.

 

Ok. I've just looked up 'IAT' to find its' location, and discovered that the IAT & ECT are one in the same.
Since the ECT has been replaced, then the IAT is a non-issue.
This is a non-turbo car.
Please correct me if I'm wrong.
Thanks.

 

Fast-forward 2 months...after changing upstream O2 sens (creating code), ECT, plugs, I'm back to square one. With the cooler weather upon us, the S60 still shows a very rough idle between 1100-1300 with some addtl surging upon initial cold start-up.
As stated prev, leaving the key at 'start' for 30 seconds before turning it to 'crank', seems to have some affect on the trouble. Doing this causes the car to operate more smoothly. I believe that the O2 sensors are being pre-heated by first leaving the key in start. However, why SHOULD this have any affect when the car runs in open loop at start-up?
Along with all of this, I can hear some kind of a motor or device which seems to be searching for a set position as the key is initially turned. I don't recall hearing this device cause so much noise previously. There is a good 4 or 5 seconds worth of 'searching' going on.
Any help would be appreciated. Thanks.

 

An update...we cleaned the TB & the S60 now performs like it should. There must have been gum at the edges of the TB as the throttle plate meets the TB. Access to the TB was as simple as removing the plastic intake hose.
I'm sure we've saved at least $400 by avoiding the 'service' dept.