1989 steering problem

A friend has a 2500 suburban with 6.2D, the problem is darting steering after a normal low speed turn, and then accelorating up to speed the wheels just seem to have a mind of there own on occation. The truck will just turn left or right and a major steering wheel correction is nessasary. So far we have replaced the steering box with rebuilt unit, left side ball joints, new steering stabilizer, and left rod ends. The condition is bad enough the the truck is not being driven. Anyone have something like this before??? Bill