hi, my tl idle goes up and down sometimes when im at a red light. i replaced my spark plugs timing belt water pump less than a year ago. and this only happens sometimes. it drops from 900 rpm to about 300 then goes back up to 900. it happens about 4-8 times. what do you guys think the problem is. im going to look through the motor today. any suggestions?

 

Idle air control valve located underneath the TB. Common problem on the 99-03 tls

 

in case anybody else stumbles upon this thread, I'd like to also point out that loose wiring in the plug to the IACV (which is located at the bottom of the throttle body) will also cause this issue. Just one loose wire is enough to cause bouncing idle but not enough to throw a check engine code like you'd get if you completely disconnected the electrical connector.