Odd feel in gas pedal

2001 Dodge Dakota, 4x4, v8.

While driving back from the place that sells truck caps, I finally
quantified a feeling I sometimes get in my gas pedal.

At about 61mph to something over 65 but under 70mph, there's this
rhythmic feel, like a cylinder about half covered with sandpaper.  About
once a second, the sandpaper passes beneath the gas petal.  Does that
make sense? A rough feeling that comes and goes about every second.
Definitely slower than tire rotation.  Because it's in such a narrow
speed band, I can't really tell if it speeds up when I do - it sort of
fades away.  The feel when slowing down doesn't so much fade, as just stop.

It's definitely not the road, though, I was on asphalt, and went from
old to brand new with no change in the feel.  Engine doesn't seem to
care, doesn't sound any different.

Appreciate any feedback.

jmc