rough idle on 2003 Mercury Grand Marquis

Happens every day.Stopped at traffic light,seems thst it wants to stall,but doesn't.Driving about 40 45 mph it sort of jerks and goes away.I har it tested diagnostic circut Check engine light.Engine light came on periodiclly and went off.They said everything was working alright.I forgot to ask if they road tested.
Thank you,

Don hochberg

Asked by for the 2003 Mercury Grand Marquis
On mine it turned out to be a bad "Idle Air Control Valve". Very easy to fix yourself and the part costs only $50-$70. Details on how to do this repair are already posted on the web so I won't repeat them here. Hope this helps.
2 answers
How many miles. A tune up is recommended at 100,000