1997 Chevy Lumina Engine Shaking

my lumina will start up in the morning and drive fine untill it heats up to operating temperture, probaly about 10 minutes after its at operating temp i will start getting shakes when i try to accelerate, they start weak and very short bursts, but the longer i drive sometime i cant even accelerate at all, my car will just shake violently at say...15 mph, i changed the spark plugs, wires, coil pack was tested, its fine, removed catalytic converter, new head gasket, lower intake gasket, cam sensor...and i think thats it, please let me know this is getting bad