2002 Buick Century Tries to stall when cold and more!

I have a 2002 Buick Centure with approx. 80,000 miles and recently it's been acting very strange. When you start the car it seems to lose "something" as it will idle down to just about stalling then snap out of it. If you just leave it alone, it will do this about four times and then it will remain running. This is on a cold start. After taking off down the road it seems to shift hard, and then when you slow down or stop it idles very high like it's going to blow, you have to shut the car off and restart it to get the idle back to normal (at this point the car is warmed up, so it does not try to stall like it does when it's cold). Any advice from the experts?