[Dad's72]

I do have a question for y'all since this site always seems to have good advice. Not always but sometimes at red lights I notice that he idle is a little rough. And by rough it is pretty dang light really. It is only noticeable in the steering wheel and pedal. If I let go of the wheel it is barely noticeable. What could this be? I thought it might be cheap gas so I filled up with premium before heading to the coast and burned through it. Filled up and did it again. I got 22 mpg on those tanks so it ran fine. Just the idle.