Volt gauge spikes

I'm trying to figure out what is going on with my truck. I'll be driving my truck and after driving abit I notice my gauge is spiking from time to time, figured it would be a loose belt, I'd tighten best I could, need to get an way to do that but seemed to work then, bam! its spiking then turn the headlights on, they start going dim then brighten up and the gauge would be spiking. So any idea's my truck has a 302 ford it has a gm alternator, c-4 transmission I know few people running the same set-up.. Thanks in advance

 

Sounds like the alt is intermittently not charging.
Brushes could be worn.