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One possibility is that the heavy red wire from the B+ terminal of the alternator to the + terminal of the battery is grounding where it runs under the crankshaft pulley. There is lots of oil, heat, vibration and sharp corners to damage insulation and cause an intermittent ground in this area. When there is a ground, the heavy load on the alternator will cause dimming of everything and a drop in voltage. It is not easy to spot problems here because it is a tight area, but with the belly pan off, you can get a bit of a look from underneath. You could try adding some additional insulation over the wire bundle to eliminate the possibility of the problem being here.