If this happening at any engine or vehicle speed, look for damaged wiring harnesses, expecially those that appear to be tight. Those runnig near an engine pulley, or over a pinch weld. and near steering components. Check engine ground cable to chassis, and check engine mounts for wear.
If no damage is found, does the light dimming correspond to the AC compressor engaging? You might have a failing AC bearing on the pulley. It would not hurt too release the serpentine belt to check the condition of the AC ulley bearing for noise, for excess radial play or other indication of pending failure such as difficulty in rotating the pulley.
If that checks out- load test the alternator (the regulator is part of it so it is tested at the same time)
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