My truck used to do this also. My alternator developed a bit of a squeak so I took it to a neighbor that rebuilds alternators and starters for several of the local shops. I figured a bearing was going bad in it and just needed to be swapped out. Instead, he said the commutator was doing the squeaking and the bearings were fine. I asked him to make it "like new" and $80 later I have my rebuilt alternator (which hasn't done the lights dim yet). He went ahead and put in new brushes, a voltage regulator, and a couple other small things. But the new commutator was the bulk of the expense at around $50. It was still cheaper than a $200 rebuilt from a parts store.
I need to state this though: I cannot prove this was a "cause and effect". But it seems like more than coincidence that my truck used to dim the lights for a few seconds while driving almost every time I drove it and in the two months since I put the junkyard and later my rebuilt alternator on it hasn't done it a single time. I didn't replace any other belt-driven accessories in that time.
While I had him rebuilding my alternator, I swapped in one I picked up from a junkyard so I could still use my vehicle. Do you have a buddy or a junkyard nearby you can try?