S500 idle problem

[trintrick]

Sometimes on startup, usually when hot, my 95 s500 idle goes up to 1000 rpms then down to 0 and dies or stumbles back up to 700 rpms where it idles normal. I was told I need an idle speed actuator. What test should be done with the tester (HHT) to diagnos this problem or what should I check to make sure this exspensive part is the problem. There are no warning lights displayed while the engine is running. Thanks.