Why will my 2000 Chevy Silverado will not always idle?

Sound like its a fuel or spark problem. I would first try to eliminate fuel. Check to see if you have fuel prossure by relaesing the fuel pressure. On the fuel line in the area of the injectors there is a valve that looks similar to a tire valve. Press the center in to release pressure. start the vehicle turn dack off the realease the pressure again. if no pressure than you need a new fuel pump. if it checks out then look at the spark . This step depend on if you have 1 ignition coil ir Individle coil packs. If you have 1 coil pull a wire from the coil and put the end of a screwdriver about 1/16th of an inch away from the metal nipple on the coil and have some one crank the engine and watch for spark. It should be a bright blue color. If light in color you have bad spark and need a new coil. Do the same at each of the wires on the spark plug side to test Dirtributor and or wires. If you have individual coil packs then pull off each coil pack and test individualy. Simalar procedure as single coil.