How is the ground hooked up your truck? If it goes to the body first, then off the body to the starter, there is a real good chance the starter is trying to hunt a ground through the starter lever.
My suggestion if you have it this way, would be to go from the battery to the bell housing (starter mounting bolt) and ground there, then run another ground off that to the body.
The more I think about it, the more I think it has to be a poor ground problem.
Keep us posted.
