Electrical ??

My voltage regulator did a melt down yesterday, my son put on a new voltage regulator,new alternator belt since the old one was stretched so far out there was no adjustment. I left to go pick up my son a little while ago everything was nice and bright. When I stopped outside the building where he was at I noticed everything got dim and so did the engine (this kept cycling like this until it was under load). The belt was tighten properly nice and tight with about a 1/4 in play in it. Any idea's of what it could be?