Could someone who knows about cars tell me why mine overheats?

Hi, I dont know too much about cars. but i do know i just bought this car in december,took it to the cadillac dealer the next day because it started overheating immediately. they put a new water pump and thermostat on the car. i was charged $1014. problem was fixed. later on the radiator started leaking. took it to another shop they put on a radiator, NOW its running hot, i took car back, they say they had to burp it because of air in system. it worked for a short while then started running hot again. now they wanna act like Cadillac dealer put defective part on my car and told me to take it back to them, but note it wasnt doing this until this shop put on radiator. no leaks or steam, only when i take off radiator cap all the coolant gushes out. even if the car sits all night, it like the antifreeze is sitting in the coolant resorvoir waiting to just burst out. and when the car starts to run hot, the anti freeze is usually NEVER HOT! im gonna lose it if i dont find out whats wrong with this car. can someone Please help a young lady out?