why does it shake horribly off and on while driving or while at a red light? on 1994 Mercury Sable

it also shifts hard when shifted from park into drive

2 answers 2 comments
poss misfire or other issues seek a diag and est to confirm
ok,thnx.
these were bad for a vacuum leak on the vapor hose from intake. runs from rear of intake, under air cleaner tube toward front of engine.
uummmm...???? ok, I guess ill be getting a book so I can figure out what and where that it... :)