Erratic timing on a 89 4.3L chevy pickup

the truck has erratic timing, no power, and back fires. would anybody know what could be the problem

Had the same problem - turned out that the distributor had problems - the plate (where the rotor is attached and the counterweights/springs are mounted) was loose! It is splined internally and is staked down on top of the distributor shaft - it loosened, and had about 1/8" play from one end of the movement to the other! Replaced it and problem went away!