Thanks much for the input - this seems to occur around 1200 to 1500 rpms when the motor is heading back down towards idle after letting off the throttle.
What rpm should my motor idle at? I have the 6.0L.
The vehicle starts fine, and it is getting pretty cold here - so I think the battery's okay.
Could it be a headlight relay problem? Though I haven't tried extensively, it has not seemed to occur with the lights off.
Thanks,
Jon