I have a 1989 chevrolet k 1500 4x4 truck with the 4.3 L TBI Engine. When accelerating, the truck seems to stumble/hesitate. It almostfeels like the truck is bucking up the road. It will do this if you hit the gas hard, but stops when you let off. Put new plug wires on about 2 months ago. any idea what the problem could be??