So far it hasn't calved, but this is what's going on and maybe you can give some idea of the problem.
It's a 1991 GMC Sierra 1500 with the fuel injected 305.
Has a four speed (3 speed with overdrive) automatic transmission.
When in overdrive and not necessarily under a load (at highway speed), the truck jerks/jumps intermittently and not often.
This is not engine related.
Any idea's?