1993 Chevy Truck missing and bucking

1993 chev. Truck v-6 running bad going down the highway, itll be running fine then all of a sudden go to bucking as if you were turing the key switch on and off super fast, I changed the i.C.M, the timing chain, I even cut out the cadilytic converter, still doing same, now its got to where when you stop alot of times it dies then also, I mean dies fast like killing the key, then it cranks right back up, has anyone else been through this