Check the spark plug wires as they may be breaking down. I bought this car used, but in great shape. I was changing the plugs when I discovered one is stripped. The person doing the damage must have threaded the old plug in with something else in the hole, because the plug didn't blow out like it does now. It was leaking, though, because there was carbon all over the threads and out onto the white insulator on the outside from blow by.
Problem is, it is the cylinder closest to the steering wheel - on the backside, naturally.
Not easy to see or access. Each repair job is different depending upon ease of assess to the damaged spark plug hole. Kits are available for thread repair. Most auto parts stores have them. I have heard that GM only wants the platinum plugs installed in this engine and this is the cause of lacking performance.