Man I fixed a bunch of trucks with this problem back when that truck was almost new. Yank the blue wire out of the spark knock sensor located on the right side of the block, it threads into the water jacket then go for a road test. If your problem is solved reset the timing to top dead center and let it rip. Chevy had a lot of problems with that electronic spark control system back then. There was a bulletin on it where you remove two of the four wires at the distributor and jumper two of them together. Just unplug the knock sensor and see if that works.