While on vacation out west, I did manage to get above 70 mph.
1999 S10 pickup, 4 cylinder automatic. 37K miles.
No air conditioning at the time.

The headlights, dash lights, all miss a beat.
This only happens at or above 70 mph.
I can't predict when it will flicker, except that I know it must be 70 or
higher.
Going up a hill, at lower speed will not put the strain required to flicker
the lights.
It has to be level ground, above 70.
So it doesn't seem to be a WORK load issue.
I may also be happening during daytime (s10 headlights are on all the time)
But I can't tell due to ambient light.

I can even feel, the tiniest flicker loss of power.

The duration is just a flicker. This will occur during acceleration or at
steady speed.
(Truck has cruise control). The flicker will occur whether I'm using cruise
or not.
At first I thought it was my eyes getting tired, but passenger confirmed it.

I've tightened battery clamps, that cable that goes below the battery holder
to some electronic device.
(don't know what it is)
Both positive and negative connections, I even changed the battery thinking
it was a bad plate.

Chevy dealer said the charging system was ok.
But as expected, they refused to go blasting down the Dan Ryan at night to
see if I was crazy or it really was flickering.

So does anyone have a suggestion?