It's done it since I got the car..
anything below about 1100rpm (Idle is around 800-850 when warm) everything in the car dims, dash lights, headlights, the heater fan slows down if its on, etc.
I didnt care until I put a power inverter in the car and it beeps when voltage drops below 11v or something around there I think. It does it almost every time I stop at a red light or stop sign and its darn annoying

I've had mixed responses from possible slipping belt to bad battery to bad alternator to bad ground[s]. I'm just wondering if this is a common issue and if there is a "most probable" cause for this so I know where to start.
I haven't even checked to see if the belt is slipping yet even though thats an easy one - I just haven't been home during daylight recently working 12h days

