PLEASE HELP!

I have a 2000 cadillac deville. When the engine is cold. ( Hasnt had a chance 2 warm up) The car seems to not change gears right or idle right for that matter. Then as the car warms it seems to run better. Changing gears smoother but still not always just right. The other day I was tryn to get on the highway and it seemd to have a lose of power. So I was almost gunnin it. When I got to where I was going I had to paralell park. When I changed it to reverse it made a loud clunk and semmed like my whole engine was gonna fall out? Same thing when I put it back in drive. I parked and checked the fluid. Iw was good. Then, when I left a couple hours later it was driving just fine? Its got new spark plugs and everything. What could it be? PLEASE HELP!