mudslingertwo

I don't know if I am in the right spot so tell me if I,m not. I have a big problem and need help. I'm working on a 94 camaro with a 3.4 5 speed that when you start it up cold and drive it you have no problems at all no matter how long you drive, or hard, sunny days or rainy days nothing happens. But as soon as you shut it off smoke a cig. or whatever as long as it is whitin 2 hours you take off driving after about 1/4 mile it starts backfiring into the intake and loseing power and a loud tic comes from the center back of the engine and it is not constant with the crank time or the cam time. To me it sounds like two hammers hitting together. I don't think it is a lifter because it has no noise at start up hot,or cold. I don't think it is a valve because it only does it after you shut it off and start it back up. I have changed the EGR valve, the coil pack, a fuel line that was leaking, and plug wires. Please help anybody or tell me where to go to get help.
thanks