200SX help

I drive a 1997 Nissan 200sx SE-R, it has 115,000 miles and it is a manual.

I was driving to work this morning on the interstate this morning, and after 30 minutes me battery light and brake light were flickering at sppeds 65-75 mph, When I got off the highway and reduced my speed to 55-60 mph this stopped, does anyone know what is going on and how to fix it. Thank you.