car revs up in park

Any ideas on what this might be:

A friend of mine has a 1998 Ford Taurus. The other day when she pulled into the drive way, put the car in park, and after that the vehicle began to rev up to 3rpms. She put the car in reverse, drove forward, then put the vehicle in park again. And again it reved up to 3rpms. We tested it a few more times and it did it once again. We continued testing it in this manner, but it stopped doing it. I checked the throttle and accelerator cable to make sure it wasn't getting snagged on anything, but it appeared to be fine.

 

The IAC can get dirty/gummed up.

 

IAC valve. I will take a look at this today. Thanks.