Electrical issue

[Sonicman]

I've been noticing that my new head lights and dash gauges will go from reallly bright to dim and back while driving. The heater fan changes speeds at the same time the lights are going from bright to dim.

I know very little about electrical stuff. I do know the battery, starter and alternator all have less than 400 miles on them. I assume the voltage regulator is the original. The truck is a 69 c10 half ton 2wd with a 250 L6.