vibrations...
 

after the car has been driven for a while, right before i come to a stop, the rpm's drop down to about 500 and the engine and steering column start to shake. then the rpm's go back up. sometimes while im stoped the rpm's keep going up and down. any suggestions?
:)-Smiley

Sounds like the IAC (Idle air control) valve is dirty and is having a hard time getting the car to idle. Either take it off and clean it or replace it.

what year is the car? how many miles, what kind of transmission, and what modds are done on the car? when is the last time you changed the oil/air filter/spark plugs, etc?

true.

its an 03' with 94,000 miles. only mod so far is a CAI. it was just paid off so im starting to mod it.

automatic and ive replaced all sparkplugs, changed the oil and filter,

I take it is a v6 correct? If so here is a photo of where the IAC is. Just unbolt it and clean it with carb cleaner and see if that helps out. If not you might have to replace it.

http://www.3.8mustang.com/forum/atta...7&d=1170539988