Lurching truck

[nosoup4u]

I have a 1968 C50 with a 327. This is a grain truck with a 4 speed and a two speed rear axle.
The old girl starts easy and runs good, but quite often when rolling down the hiway, after going through the gears and just getting to cruising speed, it begins to lurch almost like you were quickly stepping on and letting off the accelerator pedal.If you let off, and slowly speed back up it will come out of it.
It will happen if the the truck is loaded or not and in any of the higher gears.
Any ideas what might cause this?
I'll hang up and listen.