There are seven different things which usually cause this problem:
1> A mass airflow sensor which needs cleaning will do it. Be sure to only use mass airflow sensor cleaner spray to clean this part.
2> A fuel filter which has become clogged will do it. So replace the fuel filter.
3> A throttle body that needs a thorough cleaning will also do it.
4> A defective crankshaft position sensor, or a defective camshaft position sensor will often cause this type of problem.
5> The use of low quality fuel; or fuel which has large amounts of ethanol additives will sometimes do this. Chevy V-8s are often unusually senstive to fuel quality.
6> Worn spark plugs can also do this; as can spark plugs which are an unsuitable choice for this motor.The original spark plugs in this motor were Iridium. If they were replaced with a cheaper plug design; it might lead to unstable running. Depending on the plug type, fuel quality, and the type of driving the vehicle gets; it might be necessary to replace spark plugs more often than the manufacturer recommends. I would recommend Champion Iridium # 9405; at .060" gap.
7> Some Chevys have had problems with the ignition switch going bad.