2001 Silverado 2500HD Misfire issue

PROBLEM!!! I have an issue with my truck. WHen I start it up and let it idle, it starts stumbling and running really rough to the point to where it wants to shut off. The check engine light comes on and the code reads multiple cylinder misfire. If I dont shut the truck off I can put my foot in the gas and it stumbles for about 30 seconds and smells like raw gas coming out of the exhaust. I can shut the truck off and start it back up and the truck runs ok. Can someone please help me figure this out. I need some ideas as to where to start. I replaced the plugs and wires. But if its a multiple misfire then I am thinking it is maybe a computer problem. Please help. Thanks everyone.