Rough idle

Do you have a check engine light?

Try cleaning out the idle air control valve, and check the TPS/MAF/MAP sensors

Do you know which cylinder or cylinders missfiring?SES light on?Have you switched coils or plugs to see if the missfire on that cyl changes to another cyl. Need more info.02-06 envoy and t-blazer had a very common problem for leaking or stuck intake valve or valves. I've replaced alot of cyl heads on these vehicles. Very very expensive. Perform a cylinder leak down test to determine if this is the problem, which it sounds like.