Engine vibration shaking whole car

89 chevy camaro, 8 cyl. 305 throttle body. Car starts up as normal, sitting in park, engine runs about 30 seconds and a vibration starts to shake the car. There is no odd noises in the motor associated with the shaking other than the fact you can tell its missing not wanting to run correctly. In reverse and in drive it is also vibrating, the shaking is less apparent in neutral. While im driving and come to a stop sign I put the car in neutral, as it sometimes wants to die. At idol and coasting the vibration is there, upon acceleration it goes out of vibration. New spark plugs, and wires, motor mounts are fine, injectors spray fine, egr fine. Timing was set 6 months ago by a garage. Could an addtive in the oil be messing things up, as last year I put in a fuel additive to my gas tank and 2 weeks later my fuel pump went out. My boyfriend isnt much help in all this he ust doesnt want to mess with it, leaving me to deal with it. I dont want to sell it cause its my favorite car. Any help would be appreciated.