First off, it's an 05 Impala, 3.4L engine with 155,000 miles on it. When I got the car, it had been overheated one good time and parked until I got ahold of it. I flushed the system, changed the water pump radiator cap, the thermostat, and the temperature sending unit. It took a lot to clean that system. Then I changed the plugs and plug wires. It had prior damage to the catalytic converter and the resonator apparently, so I had those replaced along with the front O2 sensor because that's what the codes read. After having those replaced by a mechanic, he ran the codes again and it threw multiple random misfires. I reset the battery and have drove it about 50 miles off and on since having them replaced (converter, sensor, and resonator) and no codes, no check engine light, but it doesn't like to start, and idles very hard now for a minute until it levels out the pressure (I guess?) It is now shifting really hard and sometimes taking "a deep breath" before shifting. On top of it all, it now smokes white out of the exhaust when it's first started. It's not a lot, and it doesn't smell like coolant. Since flushing my system, (about 1000 miles) I've had to add about 2 cups to the overflow tank of coolant just to keep it at the line. I know that's a small leak, but I can't find it. It's not external. With the exhaust leak before I changed the converter, I never had any smoke and it would only shift hard (and definitely not as hard as it is now) after getting up to temperature and then having to stop and go in traffic. It's not running hot at all, and all the replacement parts are working properly. What more is there?! Help!