air flow meter jumps to my mind..
very subtle 'unevenness' .

if it has not had a thorough careful wheels-in-the-air underneath
inspection recently ..
since it seems like drivetrain looseness ....that would be good to do
again..
and even then sometimes things are hard to find without dissembly.

my vote at the moment without more info is AFM.

On 5/15/2013 4:11 PM, JRodgers wrote:
> On a 90 Vanagon, the auto tranny runs smooth as silk on acceleration -
> no matter how fast or how slow. Smooth as silk slowing down. But at
> steady speed there is a little back and forth jerking, like there is
> wear in the drive train somewhere - but I don't think that't it.
> Everything external seems tight.
>
> Anyone got any ideas??
>
> John
>