Overheating, trouble starting, idle issued

Hi, I own the car listed above GLD model. Since I bought the car second hand I have had basically the same issues. I changed water pump, radiator', plugs and wires, coil pack, camshaft position sensor, throttle position sensor and thermostat four times (not sure it was bad every time), have put another Cadillac cover her and a different exalt on it. My problem still after all that is that shortly after starting the car it will begin to overheat. When I start it I usually have to hold crank for 20-30 seconds let off then crank again feathering gas pedal. Accelerating will cause car to overheat at much quick rate but if let off the gas temperature will slowly decrease (heat has to be on at least level 3 for fan setting). Also begins to have idling issues. At red light, stop signs or any period of time car drops below 30 with little or no acceleration. If you manage to keep car running once you begin to accelerate from slow or stop position with pedal all the way to floor you might be lucky to go 2 mph. Dropping car in 2nd sometimes help or put in neutral then revving it up which snapping it into gear will get it going.
Can someone please tell me what is wrong with my car. Local garages and the places previous owner took it to can't seem to figure out what is wrong and I'm tired of throwing money into it not fixing the problem.