Engine revs during idle

I have a 1994 Ford Taurus, 190K miles, 3.0L engine. It runs fine, but when I start the engine, it goes into a rhythmic behavior while idled. The engine will start to wind down, almost as if it is going to stop,
then it will rev back up again, only to slow down. It does this continually without ever quite shutting off. As soon as I put the car in drive or reverse, the behavior quits and the engine performs normally.
The pulse is about 2 seconds in each direction. Does anyone have any idea what this is and what it means?
Thanks, hbchrist
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How about after it warms up? If it doesnt do it warm, Go to NAPA, get idle speed control valve... about 50 bucks
About 20 minute job, 3 screws, on on the throttle assy cover, two on the ISC.
If it does it warm as well, look for a small vacuum leak
Actually if ISC's never been changed, probably a good iddea to change it anyway.
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Backyard Mechanic wrote:

Thanks for the info. I will get to it this weekend, and report back.
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