navigator shakes at stops and accelerations.. on 1999 Lincoln Navigator

Coming into N.M. my check engine light turned on, there seem to be no issue, but wanted to check it out.
I took it to goodyear in alburquere, did diagnostic and said they need to fix a small pvc valve on the vacuum hose and #2 spark plug and coil. the light went off..continueing towards Taos, my suv started to shake at stops and as i accelerate and sometimes as i go past a certain speed. Its worse than before i took it in to goodyear. In May 2010, i had a aurora goodyear shop fix all coils and plugs..would a plug/coil go out so soon?
What could it be so i am aware of the next "diagnostic" test with another shop?

Asked by for the 1999 Lincoln Navigator
Take the car to a Ford specialist who can diagnose your car. It's not uncommon for these vehicles to experience airflow sensor (MAF) problems. A good diagnostic technician should be able to help you. Here's a directory link for you: