you could possibly have a bad diode in the alternator. It will result in normal function of most of the electrical system but may exhibit some "spikes" that will cause flickering of the lights which is most noticeable at idle and low speeds.
I had a problemwith the lights flickering and a new alternator fixed it.
1990 190E 2.6
1996 Grand Voyager 3.3
1985 Mustang GT 5.0 5 SPD
1982 Suzuki GS 750T