1992 Pontiac Bonneville

Engine Performance problem
1992 Pontiac Bonneville 6 cyl Front Wheel Drive Automatic

engine idles rough and has reduced power overall. Gas consumption has increased. Engine backfires when accelerating from a stop. Engine stalls after deceleration, especially rapid deceleration. Increaed engine noise thru exhaust. Replaced very fouled plugs and fuel filter after using injector cleaner. Only minor improvement. Noticed that one of the six terminals on the coil packs was slightly corroded while all the others were shiny. Cleaned terminal and got slight improvement. Seems to have plenty of power under higher rpms but not as much as before and not as smooth and quiet.
April 14, 2010.

Has the check engine light come on? Have you checked to see if the catalytic converter is plugged?