it was somehow "normal" back in the days, now with aged conections and wires, even more. A low charged batt will make that even worst. With engine running the dimming on these is reduced because the increased voltage up to 13-14 rate. In some cars you can even notice the radio volume going up and down while blinking because on these the Radio feed is shared by the tunring flasher

Hazzards are a big suckers.

As I allways say, if you make mantenience on brakes and fuel system, electrical needs that too after 50 years. But, nobody uses to care that and think electricals will survive forever.