I start truck in the morning and when I give it throttle at idle the headlights will dim and volts will drop heater fan slows. Let off throttle and gauge goes back up. The needle stays in acceptable range but it doesnt make sense that you give it throttle and the needle lowers along with lights. Is anybody else seen this. Also recently I have noticed it does it sometimes when the truck has been driven.

 

How cold is it where you are? Probably just the intake heater cycling

 

I agree with his diagnosis.
For more info, look here.
https://www.dieseltruckresource.com/...d.php?t=121337