2002 Mercury Cougar V6 dash lights dim while idling and car jerks while stopped.?

I have a 2002 Mercury cougar 2.5L V6. When I am stopped at a light or sign the dash lights dim and the car actually jerks and feels like it is losing power. I have replaced the battery, spark plugs and wires. I just had a transmission rebuild 2 weeks ago. The car also jerks like it is losing power while I am accelerating. I have heard it is either the coil packs or alternator. Does this seem like a reasonable conclusion? Thanks for the help.