Need some help

I have a 3.3 Fuel Injected Cutlass Ciera w/o carburetor, and it accelorates very slowly and the engine tends to feel a little hotter then it should, sometimes it smells like a burning metal smell, and sometimes it smells like gasoline, also tends to shake a little bit when it gets to about 60 - 70 mph. I recently changed the fuel filter, spark plugs (gapped them properly), put some fuel injector cleaner into the gas, checked all the fluids and they are fine, had a diagnostic check and all the codes came back fine, nothing wrong (twice). Not sure what it could be, thought it might be the computer, distributor, vacuum leak, or clogged fuel injectors; can anyone help me?