[71GMC_3/4T]

Check your grounds, at the headlight, at the turn signal, and also at the side marker light. The side markers are wired so that they flash with the turn signals, I think. One common place for bad grounds is at the lamp socket itself. If the outer shell of the bulb base isn't well grounded, it won't work right. Usually, you can tweak the sides of the socket in a little and get it to work better. Also, a small wire "bottle brush" works wonders to clean up the sockets and the contacts. Be sure the lights are off though, or you will toast a fuse or the socket.