2004 Chevy Lumina Shakes and Vibration

Hey, I have a Chevy-Lumina SS (LS1 engine 2004) and lately I have encountered this problem which at first was barely felt but then became a consistent issue. When I turn on my car in the morning I wait for 4 minutes, then I take off. As I take off my car shivers and vibrates a lot and my rpm gauge drops to 0.5 and my accelerator does not respond immediately. I kick it all the way and still nothing happens and it feels like my car is going to switch off but it doesn't. A few seconds later after trying, the engine responds and the car takes off normally. At 50 km/hr and above my car functions just fine. Its just the take off from stand still that bothers and the hesitation and vibration from the engine. I have changed my spark plugs and wires and fuel filter and had my air filter cleaned. Could you please tell me what other reasons might be causing this problem before I get ripped off at the authorized dealers?
also could you tell me what a major tune up means and what exactly is involved in the process. Thank you