I am hoping someone can shed some light on particularly difficult problem.

When driving on the freeway for a while, I have an abrupt bucking/misfire.
It is very specific in that it happens only when I slightly let off the
accelerator and will go away for a few minutes if I press the accelerator
firmly  or release the accelerator.  Sometimes this behavior will continue
until I stop the car or may simply stop the behavior on its own.

The only discernable pattern is that if the road requires acceleration and
deceleration, the problem doesn�t surface.  If there is a steady constant
speed, and I slightly lift the accelerator, the behavior begins.