99 expedition lights

What might cause the lights to dim? When it's idling you can see the headlights,dash etc flutter. The rpm's vary slightly in idle and the light flutter but not in the same sequence as the idle. Had the shop do a load test as well as other battery,alternator tests and of course all was ok.

I would check to see if the Idle Speed Control Valve is working as it is designed to. If it is not working properly, it can allow the idle to fluctuate and cause what you are describing.

Do you know if the shop checked for AC(alternating current) alternator output. Alternators use whats called a rectifier assembly which is composed of diodes to change AC to DC(direct current). When one of these diodes malfuntions it causes the symptom you mentioned. Although not always the case it is a possibility. You might also want to check battery terminals, cables and alternator connections, as well as engine grounds and give them all a good cleaning its something thats very neglected and sometimes cleaning them can even boost the V(voltage) that reaches the battery from the alternator.