If it's misfiring at idle on up, it's usually the injectors, check with your dealer on special policy on the injectors in your state, they can be cleand with nitrogen and them a cleaning solution. If your engine misfires under load, it's the valves, sticking in the valve guides, you'll need to pull the heads and you'll find the valve guides are machined too tight.
Check these after checking the basics, like tune up, compression check etc, the sticking guides can be found with a vacuum gauge, to i.d. which cylinders you'll need a Tech 2 the factory GM scanner.
Sorry to tell you the valve guide issue is not a special policy, one fix another guy at a GM dealer told me to try if you don't want to pull the heads is run synthic oil changing it every 1K miles, I've never tried this and have no idea if it works.
Last point, if you put aftermarket tuneup junk in your engine, that can cause misfires too, only used A C Delco parts on tune ups.