Hi i have a 91 Brougham and it idles steady but a bit rough or shaky and if its in drive with my foot on the break like at a red light it has a bad misfire every 5-10 seconds it will go from smooth to almost stalling then smooth again. Any ideas what it could be? I was thinking spark plugs but it drives fine and has good power just a misfire in gear stopped.

 

Checking for vacuum leak is good place to start. Tune up? Anything done prior to this happening?